WO2001093383A2

WO2001093383A2 - Wavelength locking system

Info

Publication number: WO2001093383A2
Application number: PCT/US2001/016972
Authority: WO
Inventors: Leroy Volz
Original assignee: Versatile Optical Networks, Inc.
Priority date: 2000-05-26
Filing date: 2001-05-24
Publication date: 2001-12-06
Also published as: WO2001093383A3; AU2001266610A1

Abstract

A wavelength locking system is disclosed. The system includes first and second detectors, an optical detector, and a processor. The first detector measures a first power level of an input light beam. The optical filter is configured to adjust a reference wavelength of the light beam. The optical filter also outputs an output light beam that is stabilized at the reference wavelength. The second detector is coupled to the optical filter, and measures a second power level of the output light beam. The processor provides an error value in the reference wavelength of the light beam. In one embodiment, the error value may be computed by differencing a ratio of the second power level to the first power level with the reference wavelength.

Description

WAVELENGTH LOCKING SYSTEM

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefits of U.S. Provisional Application Nos . 60/243,325, filed on October 25, 2000 and entitled "Wavelocker Using Moveable Mirror in Conjunction with lxN Switch", 60/212,507, filed on June 19, 2000 and entitled "Stabilization of Optical Power Levels and Wavelengths of Multiple Lasers", and 60/207,643, filed on May 26, 2000 and entitled "Hybrid Opto-electronic Processor Family for Next Generation Optical Telecommunication Networks . "

This application is also a continuation-in-part of U.S. Non-provisional Application No. 09/684,349, filed October 6, 2000 and entitled "Stabilization of A Laser Array Module."

BACKGROUND

The present disclosure generally relates to systems and methods for producing laser beams at stabilized frequencies and powers, and more specifically, to a wavelength locking' mechanism that provides such stabilized laser beams.

An optical WDM system uses a single fiber link to simultaneously transmit optical carriers of different wavelengths so that different channels of data can be carried by the different carriers and sent over the optical fiber link at the same time. The optical signal in such a fiber link is a WDM signal because it is a combination of different optical carriers at different wavelengths. Hence, a WDM system can provide a broadband transmission and a high transmission speed. Dense WDM (DWDM) techniques have been used to increase the number of multiplexed wavelengths in a WDM fiber link by reducing the wavelength spacing between two adjacent wavelengths. In addition, a WDM system can be made scalable to allow expansion of the transmission capacity by simply adding the number of optical carriers in the existing fiber links without adding new fiber links.

To increase the bandwidth and the number of communication channels in WDM networks, the International Telecommunications Union (ITU) has proposed the DWDM system in which the separation between communication channels is only 0.8 nm, or 100 GHz in frequency. Thus, a light source for such a network must also have a very narrow output line- width. This requirement entails having the wavelength of the output signal to be concentrated in a very narrow portion of the optical spectrum. Further, the wavelength of the source must be stable to avoid drifting into the wavelength range of another channel .

Conventional wavelength lockers monitor and control the wavelength of light produced by a light source such as a laser. A laser is typically tuned to produce light of a predetermined wavelength. However, a number of internal and external factors may cause the laser wavelength to change or drift. For example, in a diode case, the driving current can change the resonant characteristics of the cavity. Consequently, the wavelength of the light produced by a laser drifts from the predetermined wavelength. Other factors, such as shot noise, temperature fluctuation, and mechanical vibrations, may also change the laser wavelength.

Wavelength locking mechanisms have been used to stabilize a laser at a desired wavelength. In some wavelength locking mechanisms, for example, light from the laser is transmitted to a collimator, and travels down a fiber. Conventional systems monitor the wavelength of the incoming light by transmitting the beam to a spectrum analyzer. The spectrum analyzer determines the wavelengths that comprise the beam of light. The spectrum analyzer transfers the information on the wavelength to a feedback system. The feedback system uses this information to change the temperature or other laser parameters to compensate for any drift in the wavelength of the light from the predetermined value. The temperature is often controlled using a thermo-electric (TE) pad or cooler.

SUMMARY

In recognition of the above-described difficulties, the inventors recognized the need for providing a wavelength locking system in which the wavelength of the transmitted light is monitored and adjusted. Further, a need exists for a system that is cheaper and easier to manufacture.

In one aspect, the present disclosure describes a wavelength locking system. The system includes first and second detectors, an optical detector, and a processor. The first detector measures a first power level of an input light beam. The optical filter is configured to adjust a reference wavelength of the light beam. The optical filter also outputs an output light beam that is stabilized at the reference wavelength. The second detector is coupled to the optical filter, and measures a second power level of the output light beam. The processor provides an error value in the reference wavelength of the light beam. In one embodiment, the error value may be computed by differencing a ratio of the second power level to the first power level with the reference wavelength. In another aspect, the wavelength locking system includes optical filter, a power detector, and a control element. The optical filter operates to adjust a reference wavelength of an input light beam. The optical filter also outputs an output light beam that is stabilized at the reference wavelength. The power detector is configured to measure first and second power levels of the output light beam at the reference wavelength and at an offset wavelength away from the reference wavelength, respectively. The control element provides an error value in the reference wavelength of the light beam.

In a further aspect, the present disclosure describes a method for locking a reference wavelength of an input light beam. The method includes first detecting the input light beam to measure a first power level of the light beam. The method also includes adjusting the reference wavelength of the light beam, and outputting an output light beam that is stabilized at the reference wavelength. The output light beam is then detected to measure a second power level of the output light beam. An error value in the reference wavelength of the light beam is computed.

BRIEF DESCRIPTION OF THE DRAWINGS

Different aspects of the disclosure will be described in reference to the accompanying drawings wherein:

FIG. 1A illustrates an embodiment of a laser array module that provides laser wavelength and power control;

FIGS . IB to ID show different ways in which a laser beam may be divided for primary purpose and for feedback; FIG. 2 shows one embodiment of an effect the temperature of a laser may have on the wavelength of the collimated beams;

FIGS. 3A and 3B show different embodiments of a switching mechanism shown in the laser array module;

FIGS. 4 to 6B show four different configurations of an optical switch;

FIG. 7A shows a temperature stabilized Fabry-Perot filter; FIG. 7B shows a power transfer function of the Fabry- Perot filter shown in FIG. 7A;

FIG. 7C shows a wavelength locking system in accordance with an embodiment of the present disclosure;

FIG. 7D shows an alternative embodiment of the wavelength locking system shown in FIG. 7C;

FIG. 7E shows another embodiment of the^' wavelength locking system shown in FIG. 7D;

FIG. 7F shows a graphical representation of a process performed by the wavelength locking system of FIG. 7E; FIG. 8A shows a laser power control servo system that monitors the power being generated by a laser;

FIG. 8B shows a wavelength control servo system that monitors the wavelength of a laser; and

FIG. 8C shows an etalon temperature control servo system that monitors the temperature of the etalon utilizing the signal derived from a thermal sensor. DETAILED DESCRIPTION

The present disclosure describes a laser array module in which the wavelength and power of the transmitted light may be monitored and controlled. This laser array module provides wavelength and power stabilization without significant interruption of the light beam emanating from the array module. Furthermore, configurations of the laser array module using an optical switch allow stabilization of a series of lasers. This configuration provides significant advantage over other configurations in which a series of temperature-stabilized wavelength locking mechanisms are used to respectively stabilize the lasers. The wavelength locking mechanisms may be costly. Moreover, the calibration of the mechanisms may be time consuming.

FIG. 1A illustrates an embodiment of a diode laser array module 100 that provides laser wavelength and power control. The laser array module 100 includes a plurality of lasers 102 configured to provide light beams 104, 106 of different wavelengths, λ_x to λ_N. The array module 100 may also include an Nxl switching mechanism 108, a wavelength locking mechanism 110, a controller 114, and a laser parameter feedback control 116. The switching mechanism 108 may be an optical switch or fiber coupler. In some configurations, a pair of photodiodes 118, 120 and a digital signal processor (DSP) 112 may be used to convert optical signals to electrical signals for DSP processing. The controller 114 may be used, in this configuration, for timing synchronization. A plurality of lasers 102 is configured to provide output light. Each laser may be individually controlled to lase at a wavelength different from other lasers. For example, the wavelength of the lasers 102 may operate at different ITU wavelengths. The switching mechanism 108 may be used to multiplex the light beams of different wavelengths from the lasers 102 for stabilization processing by a common wavelength locking mechanism 110. The common wavelength locking mechanism 110 is designed to lock different lasers at different laser wavelengths. In one implementation, the mechanism 110 can lock one laser at a time. Hence, the Nxl switch 108 switches one output beam at a time from a laser in the module 102 into locking mechanism 110 and the control loop in a predetermined sequence . The laser wavelength control loop includes the locking mechanism 110, the PDl 118 and PD2 120, the DSP 112, and the laser parameter feedback control 116. The DSP is designed with some processing intelligence and logic to determine the errors in wavelength and power levels of a laser based on output signals from the photodetectors 1 and 2 118, 120. A command is then generated and sent to the laser parameter feedback control 116. The control 116 then responds to adjust the respective laser to reduce the detected errors.

Since the stabilization processing may be configured in a feedback loop, the lasers 102 may be designed to provide a portion of the light for the feedback loop in addition to the primary output of the laser. This may be done by providing an optical beam splitter or fiber tap coupler 130 in the output of each laser as shown in FIG. IB. This may also be done by tapping a secondary facet 142 for the feedback light as shown in FIG. 1C. For example, a laser may provide about 90 to 95% of the light through a first facet 140 and provide 5 to 10% of the light through a second facet 142. In the illustrated embodiment, the first facet 140 provides light beam through the front of the laser and the second facet 142 provides light beam through the rear. The laser beam may also be tapped using an integrated grating approach as shown in FIG. ID. The illustrated configuration shows a thin prism 150 with Phase Grating 152 mounted on the prism 150. The prism 150 is configured to divide the output light from the laser 154 into two parts. The two parts of the beam may be directed to two separate output fibers 156, 158. One part of the beam may be configured to contain 90 to 95% of the beam power, while the other part is configured to contain 5 to 10% of the beam power. The two fibers 156, 158 are displaced from the centerline or optical axis 160 of the lens 162. One fiber 156 is slightly above the optical axis 160, and the other fiber 158 is slightly below the axis 160. By using the prism 150, the main beam may be deviated slightly upward to properly focus the top fiber 156. The grating 152 diffracts a selected amount of energy at an angle needed to focus on the bottom fiber 158. This configuration provides a means for sampling an output fiber optic signal to determine parameters such as wavelength, power level, and signal modulation. Alternatively, the top fiber 156 may be placed directly on the optical axis 160. This eliminates the need for a prism to deviate the main beam. The grating 152 may deviate the selected amount of light to the bottom fiber 158.

As stated above, the wavelength of the output light may be adjusted by an external means that controls the laser parameters. In one embodiment, the temperature of the laser, such as a laser diode, may be adjusted to modify or stabilize its wavelength.

FIG. 2 shows one example of the effect of the temperature of a diode laser on its output wavelength. As the temperature rises, the laser wavelength increases. This is in part due to the expansion of the laser resonator of the diode. In this embodiment, the laser array module 100 may utilize a thermo-electric (TE) pad or cooler as a laser parameter feedback control for each laser in the laser array 102. Therefore, the control 116 includes at least an array of TE pads/coolers that are respectively in thermal contact with lasers. The feedback control 116 may also use other laser parameters to control the laser wavelength.

Tapped beams 106 from the plurality of lasers 102 may be multiplexed to the stabilization processing portion of the laser array module 100. The stabilization processing portion may include the wavelength locking mechanism 110 and the laser parameter feedback control 116. In the embodiment of FIG. 1A, each tapped laser beam is switched to the wavelength locking mechanism 110 by the Nxl switch 108.

FIG. 3A shows one embodiment 300 of the Nxl switch 108 in FIG. 1A. The switch 300 is an optical switch configured to sequentially pass the tapped beams from the lasers one at a time. In this embodiment, the switch is controlled by an electronic control 302. The electronic control 302 also measures the wavelength drift and the total power. These measured parameters are utilized to generate commands for the laser parameter feedback control 116.

In another embodiment, shown in FIG. 3B, the switch 310 includes N optical switches 312 configured to be controlled by an electronic control 316. The control 316 may sequentially turn each switch 312 on and off to pass the beams from the lasers. In this embodiment, the switch 310 also includes an Nxl optical coupler 314 to couple the switched light from the optical switches 312 to the wavelength locker 110. FIGS. 4 through 6B illustrate exemplary configurations of an optical switch. FIG. 4 illustrates an optical switch designed with a moving shutter 400 positioned at the output of a secondary facet 402 of the laser 404. The shutter 400 may be controlled to move between positions where the first position allows the light beam to pass while the second position blocks the light beam from passing.

In another embodiment of an optical switch shown in FIG. 5, the moving shutter 400 of FIG. 4 is replaced with an electronic modulator 500 that modulates the light beam at least between an on-state and an off-state. However, the light may be modulated into any number of modulating states.

In order to avoid modulating the laser beams individually, the shutter 400 or the modulator 500 may modulate the output of the switch. For example, in FIG. 6A, a switching lens 604 is used to receive the output light from a plurality of fibers 606 and direct them to a switching galvo 600. The galvo 600 modulates the output light. The switching galvo 600 operates to adjust a mirror 602 to pass the output light of a specific wavelength. In the illustrated embodiment of FIG. 1A, the switching galvo 600 of FIG. 6A may be inserted in place of the switching mechanism 108.

FIG. 6B shows a variation of FIG. 6A, where a plurality of collimating lenses 608 are used to respectively convert the output beams from the multiple fibers 610 to a plurality of collimating beams 612. A galvo mirror 614 may be adjusted to pass one collimating beam having a specific wavelength 616 into an output fiber 618.

In the illustrated embodiment of FIG. 1A, the amplitude dither may modulate the light of individually selected lasers in a sinusoid pattern such that the power of a selected laser may be modulated as well as the frequency. In this configuration, power detectors (PDl 118 and PD2 120) are configured to sample the laser beams that are split from output of one laser to obtain power level signals. Since the power level signal output from the PDl 118 measures the output of the wavelength locking mechanism 110, the power level signal at that point is a filtered signal. The output signal from the PD2 120 provides a reference for the digital signal processor (DSP) 112. In some embodiments, the DSP 112 may be replaced by an analog synchronous detection block. A signal to noise ratio may be improved by doing some form of phase-locked loop (PLL) and bandpass filtering after the PDl and PD2. In some embodiments, the wavelength locking mechanism 110 of FIG. 1A may be configured with a temperature stabilized Fabry-Perot filter 700 as shown in FIG. 7A. The Fabry-Perot filter 700, sometimes referred to as an etalon, includes the cavity 702 formed by two highly reflective mirrors 704, 705 disposed parallel to each other. The distance ' ' between the mirrors 704, 705 determines the cavity length.

The input light beam 706 to the filter 700 may enter the first mirror 704 at an angle θ to the mirror surface. The output of the filter 700 is the light beam leaving the second mirror 705. As described, the input signal 706 is incident on the left surface 704 of the cavity 702. After one pass through the cavity 702, a part 710 of the light leaves the cavity 702 through the right facet 705 and a part 708 is reflected. A part of the reflected wave 708 is again reflected by the left facet 704 to the right facet 705. For those wavelengths for which the cavity length is an integral multiple of the wavelength in the cavity 702, all the light waves transmitted through the right facet 705 add in phase. Such wavelengths are called the resonant wavelengths of the cavity. The fraction of incident light that is reflected by the mirror is referred to as the reflectivity (R) of the mirror. For example, the resonant wavelength of the cavity may be adjusted by changing the cavity spacing through the change in temperature of the cavity. The change in temperature of the cavity may also change the index of refraction of the filter, which adjusts the resonant wavelength.

The power transfer function of a filter (e.g. etalon) is the ratio of input light power that is transmitted by the filter 700 as a function of optical wavelength, λ, when the cavity spacing is fixed at a constant . This transfer function of the etalon is a function of the cavity spacing (d) , the incident angle (6>) , the index refraction of the cavity (n) , and the reflectivity (R) of the mirrors. The power transfer function of the filter 700 is plotted in FIG.

7B. The widths of the waves become narrower as R increases. In a WDM system, the wavelengths for different WDM channels are evenly spaced. The spacing is usually far apart compared to the width of each passband of the filter transfer function. The spectral range between two successive passbands of the filter is called free spectral range (FSR) .

In the illustrated embodiment of FIG. 7B, the cavity spacing (d) is fixed. This allows the filter 700 to lock the wavelengths of output beams from a plurality of lasers . The wavelengths from the plurality of lasers are evenly spaced so that the wavelength spacing is substantially equal to the FSR and the WDM channel spacing. For example, FIG. 7B shows two wavelengths, λ_x and λ₂, from two lasers, which are FSR length apart. In this illustration, the wavelength of the first laser is adjusted to lock at λ_1# which is set on the side of the first transmission peak 712. The wavelength λ₁ may be adjusted to lock at a point in the curve where the slope of the peak 712 is highest 712A. This allows the dithering of the wavelength λ about Δλ_x to be highly exaggerated at the output so that the feedback loop of the filter may easily lock the wavelength at λ_x.

To lock the wavelength of the second laser at λ₂ that

is FSR apart from λ_x, the switching mechanism 108 is controlled to switch off the first laser beam and to switch the second laser beam into the locking mechanism 110. This would place the filter to operate at the slope point 714A of adjacent transmission peak 714 for the second wavelength, λ₂. Locking of other lasers at different wavelengths can be similarly configured. Notably, the above locking mechanism does not require physical tuning of the Fabry-Perot filter 700 by adjusting the filter parameters (e.g. cavity spacing) . Therefore, the filter 700 can be stabilized under the same condition in locking different lasers. Alternatively, the filter 700 may be physically tuned by adjusting, for example, the cavity spacing (d) , to change the transmission from one wavelength to another in order to provide frequency references in locking different lasers. FIG. 7C shows a wavelength locking system 120 in accordance with an embodiment of the present disclosure. The system 120 uses an etalon 716 to lock the wavelength of the incoming light 718. The system 120 is configured such that the ITU grid wavelengths are directed to pass through the transfer function (see FIG. 7B) on a particular point (e.g. 712A or 714A) on the response curve of each of the desired wavelengths. In the illustrated embodiment of FIG. 7C, as the wavelength of the incoming light 718 varies about its nominal value, the output of the power detector 722 may vary. For example, the wavelength of the incoming light 718 may be tuned to the right of the peak (e.g. at 712A or 714A) in the response curve. In this case, the output power of the detector 722 increases as the wavelength of the incoming light 718 decreases. Further, the detector 720 may be used to sample the total power of the incoming light 718. The output of this detector 720 may be used to normalize the output value of the detector 722. Thus,

. PDl value

Error = Tar_get_K - _J5 ^- . ₍₁₎

The above calculation may be performed in a processor 724.

Therefore, small variations in the manufacturing process of this system 120 may be calibrated out by testing the system 120 at each λ_n. Once the system 120 is manufactured and calibrated, the system should remain stable over time. The stability of the system may be manifested in the parameters n, d, θ, or R. As was explained above in connection with FIGS. 6A and 6B, a mirror may be adjusted to a desired wavelength of an input beam. FIG. 7D shows the mirror 730 adjusting its position to select a desired wavelength of the input beam 740. The mirror 730 de-selects other beams (e.g. those beams outside of the desired wavelength) . However, in order to tune the mirror 730 to a specific wavelength, λ_n, the mirror must be finely adjusted to a particular angle and stabilized at that angle. The mirror 730 may include a reflector and an actuator. In one embodiment, the actuator may be a galvanometer.

The fine mirror positioning may be provided to the mirror 730 by a feedback controller 736. The feedback controller 736 may receive beam position information from the detector 738.

In the illustrated embodiment, the detector 738 operates as a power detector and a quad position detector for an incoming light 740. The quad detector measures the amount of light falling on each quadrant of the detector 738 to determine the position of the beam falling on the detector 738. The focusing lens 739 may be used to focus the beam onto the quad detector.

When the detector 738 functions as a power detector, the detector 738 provides same information provided by the detector 720 to compute the total power of the input light 740. This may be done by summing the outputs of all four quadrants. Further, the quad detector 738 provides angular orientation information of the mirror 730. This information 742 may be provided to the feedback controller 736 to adjust the mirror 730 to cancel out any error detected by the detector 738. The detector 744 performs same function as the detector 722 in FIG. 7C.

Therefore, in the illustrated embodiment, the mirror 730 performs two functions. One function is the adjustment of the mirror 730 to select a laser beam of a particular wavelength. Another function is the jittering of the mirror 730 to finely adjust the position of the mirror 730 to lock the wavelength of the selected beam based on the feedback signal . In an alternative embodiment to that shown in FIG. 7D, the wavelength locking mechanism 750 may be implemented as shown in FIG. 7E . In this embodiment, the wavelength locking mechanism 750 is configured to operate with only one detector 752. This detector 752 performs dual functions in determining the angular offset of the mirror 754 and the total power in the filtered beam 758. The total power in the filtered beam 758 is the same measurement made by the detector 722 (FIG. 7C) and the detector 744 (FIG. 7D) . This measurement is reflected as the ' PD2 value' in Equation 1 above. Furthermore, the detector 752 may be configured to detect the total peak power of the incoming beam 756. This measurement provides the 'PDl value' in Equation 1. Therefore, the detector 752 should be provided with means to measure both the beam power at the filter output ( ' PD2 value ' ) and the peak power ( ' PDl value ' ) . FIG. 7F illustrates a process to provide the above- described measurements in accordance with an embodiment of the present disclosure. In the illustrated embodiment, the process may involve first measuring the output power of the etalon 760 at λ . This measurement may be made at a

reference point (θ_ref) 770 on the slope of the transfer function curve at which the laser wavelength is to be stabilized. The mirror 754 may then be adjusted in a particular direction to move the curve to a new position

( θ_ref+θ_offset) such that the peak 772 of the transmission curve

is positioned at λ . At this position, the total incident power transmits through the etalon and is received by the detector 752. Both measurements are made with the detector 752. Thus, the wavelength error may be calculated as follows :

Power I Q_ref

Error \ = Targ et - . _{( 2 )} rower i v_ref + v₀ff_set

The movement of the mirror 754 in the above-described process may be configured in more than one direction. The quad detector 752 may then detect and maintain a constant angle in one direction.

A control unit 753 is implemented to control the actuator of the reflector 754. First, the control unit 753 adjusts the mirror 730 to select a laser beam of a particular wavelength. Second, the control unit 753 jitters the mirror 730 to finely adjust the position of the mirror 730 to lock the wavelength of the selected beam based on feedback signal. Third, the control unit 753 slides the input wavelength to measure the peak power of the input beam.

There are at least four types of servo control loops desirable for the wavelength locking mechanism 110 (FIG. 1A) . The first controls the power level of each of the sourcing lasers. The second controls the temperature of the lasers. The third controls the operating wavelength of each of the source lasers. The fourth controls the temperature of the Etalon reference. In addition to the servo controls, there are calibration requirements, which must be performed during manufacture as well as during the operation of the device. These servo systems may be implemented using DSP based controls. The systems are optically integrated systems. The below described designs of the optically integrated monitoring servo systems are integrated such that the systems only need to be provided with data, input clock, and power to operate as communication devices. All the control and monitoring is performed within the system.

The laser power control servo system 800, shown in FIG. 8A, monitors the power being generated by the laser 802. The system 800 utilizes the signal derived from the Power Detector Photo Diode 804, which monitors the laser's secondary (back facet) output continuously. This signal is fed into the dedicated laser driver circuit 806, which contains a closed loop power control circuit 808. The closed loop control circuit 808 maintains the power output of the laser 802 at the reference value demanded by the system DSP 810. The system DSP 810 learns the correct value to demand during factory calibration of the wavelength locking mechanism. The DSP can measure the power of each of the lasers as they are being processed for wavelength control .

The laser temperature may also be controlled by the laser power control servo system 800. The system 800 monitors the temperature utilizing the signal derived from the laser thermal sensor 812, which is located near each laser 802. This temperature level signal is sampled by an analog-to-digital converter 814 on command from the system DSP 810, at some appropriate periodic rate. The rate may be programmed to be adequate to sustain the laser temperature at an acceptable level. The system DSP 810 then performs the temperature control based on the latest temperature reading and the commanded temperature setting. The DSP 810 may demand the desired value to the digital-to-analog converter 816. The generated voltage is sent to the TE cooler driver 818, which drives the TE cooler 820 with the selected amount of power. The temperature of the laser 802 is directly affected by this change in temperature of the TE cooler 820.

The wavelength control servo system 830 is shown in FIG. 8B. The system 830 monitors the wavelength of the laser 832 selected by the optical multiplexer 834 utilizing the signal derived from the two photo detectors 836, 838 located near the etalon 840. These wavelength dependent signals are sampled by the analog-to-digital converter 842 on command from the system DSP 844, at some appropriate periodic rate. The rate may be programmed to be adequate to sustain the wavelengths at their desired level. The system DSP 844 performs the wavelength control based on the latest sample. The DSP 844 then commands the desired change in temperature to the temperature control portion of the laser power control servo system 800. The laser 832 is affected by this requested change in temperature and the wavelength is corrected.

The etalon temperature control servo system 850, shown in FIG. 8C, monitors the temperature of the etalon 852 utilizing the signal derived from the thermal sensor 854.

The thermal sensor 854 is located near the etalon 852. This temperature level signal is sampled by the analog-to-digital converter 856 on command from the system DSP 858, at some appropriate periodic rate . The rate may programmed to be adequate to sustain the etalon temperature at an acceptable level. The system DSP 858 performs the control based on the latest sample. The DSP 858 then commands the desired output value to the digital-to-analog converter 860. The generated voltage is sent to the heater driver 862, which drives the heater 864 with the selected amount of power. The temperature value may be learned during factory calibration, and may be preserved by the DSP 858 by storing the value in a non-volatile memory device in the DSP's memory space.

While specific embodiments of the invention have been illustrated and described, other embodiments and variations are possible. For example, the laser array modules of specific embodiments as described above or other embodiments may be used in a telecommunication equipment or devices such as a line card. Further, although the optical filter is shown as stabilizing the reference wavelengths for two wavelengths, λ_x and λ₂, any number of wavelengths, λ_x to λ_n, may be stabilized by the optical filter of the present disclosure .

All these are intended to be encompassed by the following claims.

Claims

What is claimed is :

1. A wavelength locking system, comprising: a first detector configured to measure a first power level of a light beam; an optical filter to adjust a reference wavelength of said light beam, said optical filter outputting an output light beam that is stabilized at the reference wavelength; a second detector coupled to said optical filter, said second detector configured to measure a second power level of said output light beam; and a processor configured to provide an error value in the reference wavelength of the light beam.

2. The system of claim 1, further comprising: a beam splitter that splits the light beam to direct the light beam to said optical filter and said second detector.

3. The system of claim 1, wherein said first detector includes a quad detector having four quadrants .

4. The system of claim 3, further comprising: a switching mirror configured to select the light beam having a particular reference wavelength.

5. The system of claim 4, wherein said quad detector detects angular offset of the switching mirror.

6. The system of claim 5, further comprising: a control element operating to adjust angular orientation of the switching mirror based on the angular offset detected by said quad detector.

7. The system of claim 3 , wherein said quad detector measures the first power level of the light beam by summing each output of the four quadrants

8. The system of claim 1, wherein said error value is computed by differencing a ratio of said second power level to said first power level with said reference wavelength.

9. The system of claim 1, wherein said optical filter is an etalon.

10. The system of claim 9, wherein said etalon is capable of stabilizing the light beam at a plurality of reference wavelengths .

11. A wavelength locking system, comprising: an optical filter to adjust a reference wavelength of an input light beam, said optical filter outputting an output light beam that is stabilized at the reference wavelength; a power detector coupled to said optical filter, said power detector configured to measure first and second power levels of said output light beam at the reference wavelength and at an offset wavelength away from said reference wavelength, respectively; and a control element configured to provide an error value in the reference wavelength of the light beam.

12. The system of claim 1, wherein said error value is computed by differencing a ratio of said second power level to said first power level with said reference wavelength.

13. The system of claim 11, further comprising: a switching mirror configured to select the light beam having a particular reference wavelength.

14. The system of claim 13 , wherein said power detector includes a quad detector having four quadrants.

15. The system of claim 14, wherein said quad detector detects angular offset of the switching mirror.

16. The system of claim 15, wherein said control element is configured to adjust angular orientation of the switching mirror based on the angular offset detected by said quad detector.

17. A method for locking a reference wavelength of an input light beam, comprising: first detecting said input light beam to measure a first power level of the light beam; adjusting said reference wavelength of said light beam, said adjusting including outputting an output light beam that is stabilized at the reference wavelength; second detecting said output light beam to measure a second power level of said output light beam; and computing an error value in the reference wavelength of the light beam.

18. The method of claim 17, wherein said computing an error value includes differencing a ratio of said second power level to said first power level with said reference wavelength.