WO2002027873A2

WO2002027873A2 - Integrated semiconductor optical amplifier system

Info

Publication number: WO2002027873A2
Application number: PCT/US2001/029063
Authority: WO
Inventors: Jeffrey A. Korn; Dale C. Flanders; Peter S. Whitney
Original assignee: Axsun Technologies, Inc.
Priority date: 2000-09-28
Filing date: 2001-09-18
Publication date: 2002-04-04
Also published as: AU2002211238A1; WO2002027873A3

Abstract

A semiconductor optical amplifier system (100) comprises a hermetic package (112). In the typical implementation, this hermetic package (112) is a standard butterfly, DIP or miniDIL package. An optical bench (116) is sealed within this package (112). A first fiber pigtail (118) enters this package via a feed-through to connect to the bench (116) and terminate above the bench. A second optical fiber pigtail (120) enters the package (112) via a second fiber feed-through to connect to the bench (116) and similarly terminate above the bench. A semiconductor amplifier chip (102) is connected to the bench (116) to provide amplification. Isolators (128, 130) are further incorporated along with a monitoring diode (126) to yield a fully integrated system.

Description

Integrated Semiconductor Optical Amplifier System

BACKGROUND OF THE INVENTION

A common modality for optical signal amplification is the rare-earth doped fiber amplifier. These devices have good amplification characteristics and a well-understood long-term behavior. Moreover, they can be inserted into a fiber link via fiber splicing, which is a low loss coupling technique.

An alternative amplification modality is the semiconductor optical amplifier (SOA). SOA systems have a number of advantages relative to the common erbium-doped amplifier (EDFA) scheme. SOA's are typically electrically, rather than optically, pumped. As a result, they can be more efficient and avoid the need for ancillary laser pumps. Moreover, they are usually physically smaller than fiber amplifiers, which require a relatively long length of doped fiber and even erbium-doped waveguide (EDWG) devices. This quality is especially relevant when amplification is required in larger systems offering higher levels of functionality, such as optical add-drop devices and cross-connect systems, and also metro-area optical systems. The semiconductor optical amplifier can be as small as the semiconductor chip with the required packaging.

Nonetheless, the principal barrier to the commercial deployment of semiconductor optical amplifiers is the difficulty associated with coupling optical signals into and out of the semiconductor amplifier chip. The coupling issues are analogous to coupling light between a semiconductor laser transmitter/laser pump into an optical fiber twice, with additional problems associated with back-reflection suppression.

SUMMARY OF THE INVENTION

The present invention concerns a semiconductor optical amplifier system and the implementation of a semiconductor optical amplifier system on a single substrate or optical bench. As such, the present invention is applicable to problems requiring the inclusion of physically-compact amplification systems into larger optical systems.

When constructing optical semiconductor optical amplifiers, it is typically desirable to have some type of feedback mechanism to control the amplification level of the semiconductor optical amplifier. Some techniques for diverting a portion of the optical signal from the amplifier chip, however, can be susceptible to polarization shifts in the optical signal. As a result, they can introduce some noise into the feedback scheme.

In general, according to one aspect, the present invention features a semiconductor optical amplifier system. It comprises a package, which is typically hermetic. In the typical implementation, this package is a standard butterfly package, dual inline package (DIP), or mini-DIL package. Such packages are less than about 13 millimeters (mm) wide and 30 mm long. An optical bench is sealed within this package. A first fiber pigtail enters this package via a feed-through to connect to and terminate above the bench. A second optical fiber pigtail enters the package via a second fiber feed-through to connect to and similarly terminate above the bench. A semiconductor amplifier chip is connected to or installed on the bench.

In a preferred embodiment, at least one isolator is included in the hermetic package, and specifically on the optical bench, for suppressing back-reflections into the fiber pigtail and/or the semiconductor optical amplifier chip. Specifically, in the preferred embodiment, a first isolator suppresses back-reflections into the input, or first fiber pigtail, and a second isolator suppresses back-reflections into the semiconductor optical amplifier chip.

In the preferred embodiment, additional optical components are provided to o facilitate the transmission of optical signals through the system. Specifically, a first collimation lens is installed on the bench between the first isolator and the termination of the first fiber pigtail to improve the collimation of light emitted from the first fiber pigtail, in one implementation. A focusing lens is installed on the bench between the first isolator and the semiconductor optical amplifier chip to couple light from the first isolator into the 5 semiconductor optical amplifier chip. Further, a second collimation lens is installed between the second isolator and the chip to couple light from the chip into the second isolator. Finally, a second focusing lens is installed on the bench for coupling light from the second isolator into the second pigtail.

Although, in the preferred embodiment, discrete optics are used to couple light into o and out of the fiber pigtails, in alternative embodiments, fiber lenses may be formed on the fiber endfaces to reduce or eliminate the need for discrete coupling optics between the fiber endfaces and the other components of the system.

According to another embodiment, in a single physical port embodiment, the optical signal is received into the hermetic package by a fiber, focused onto the amplifier chip, reflected to pass back through the chip, and then refocused onto the fiber so that the amplified optical signal exits from the system.

In some implementations, a monitoring diode is provided that receives light from only a portion of a cross-section of the beam emitted from the optical amplifier chip, this way, a feedback signal can be generated which is not susceptible to noise from polarization changes in the beam, for example. For example, the portion of the cross- section of the beam is received by the monitoring diode by inserting a specular reflector or scattering feature into the beam path to only nick an edge of the beam

The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:

Fig. 1A is a perspective view of a semiconductor optical amplifier system according to the present invention;

Fig. IB is a close-up view of the inventive semiconductor optical amplifier system;

Fig. 2 is a block/schematic view of a second embodiment of the semiconductor optical amplifier system of the present invention; and Fig. 3 is a perspective view showing a composite mounting structure used to hold lenses 134, 136 to provide for z-axis alignment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 A shows a semiconductor optical amplifier system (SOA), which has been constructed according to the principles of the present invention.

Generally, the SOA 100 comprises a hermetic module or package 104. In the view of Fig. 1A, the top of the hermetic module 104 has been removed.

The hermetic package 104 has an input fiber feed-through to which an input fiber ferrule 112 is attached. An input optical fiber pigtail 118 enters into the hermetic package 104 via the input fiber feedthrough.

The hermetic package 104 also has an output fiber feedthrough, in which an output fiber ferrule 110 is installed. The output fiber pigtail 120 passes through the hermetic package 104, exiting from the module.

Within the hermetic module 104, an optical substrate or bench 116 is installed. In 5 the typical implementation, the optical bench 116 is installed on a thermoelectric cooler 106. A thermocouple or thermistor 108 is typically attached to the top of the bench 116 to detect the temperature within the module 104 to enable the temperature stabilization.

Fig. IB is a close-up view, better illustrating the configuration of components on the optical bench 116.

o Specifically, the input optical fiber 118 passes through the ferrule 112 and is secured to the bench 116 via a fiber mounting structure 150. This structure secures the fiber endface or optical signal source 122 such that it is terminated above the optical bench 116.

The diverging input beam that is emitted from the fiber endface 122 is collimated or 5 has its collimation improved to possibly form a beam waist via a first collimating lens 132, which is supported above the substrate on an optical component mounting structure 152. This first collimating lens 132 generates a generally cylindrical, but diffracting input signal beam, which enters a first isolator 128. The first isolator prevents back reflections into the optical signal input port/fiber endface 122. In one implementation, the input signal forms a beam waist inside of the isolator 128.

The optical signal after exiting the first isolator is focused by a first focusing lens 134, which is supported above the substrate on a second optical component mounting structure 154. Specifically, the optical signal is focused and thus coupled into a semiconductor optical amplifier chip 102. some embodiments, the chip 102 is mounted to a carrier, which, in turn, is mounted to the bench 116.

The optical signal is amplified in the semiconductor optical amplifier chip 102. Typically, these amplifier chips are constructed from AlGaAs substrates with a ridge waveguide structures. The invention, however, is of course applicable to chips made with other material systems/chip configurations.

The amplified optical signal is emitted from the chip 102 in a typically diverging beam. A second collimating lens 136, which is supported above the bench 116 on a third optical component mounting structure 156, generates a collimated, diffracting beam that forms a beam waist.

In the preferred embodiment, the mounting structures 154 and 156 are preferably composite structures that allow for alignments in the x- and y- axes, but also the z-axis as illustrated in Fig. 3. Specifically, two z-axis flexure pieces 1102A, 1102B are used to control rotation around the x-axis or in the direction of angle θ_x, thereby determining the resistance to force components along the z-axis. Preferably, the z-axis flexure pieces 102 are separately fabricated and bonded to base surface of portion 1101. Base surfaces of the pieces are then bonded to the bench with the lens being bonded to optical element interface 1112. As a result, the z-axis position of the focal point of the lens can be controlled relative to the SOA chip facets.

Returning to Fig. IB, the amplified optical signal beam then passes through a second isolator 130 for preventing back-reflections into the chip 102. The beam of the amplified optical signal, which exits from the second isolator 130, is focused by a second focusing lens 138, which is supported above the substrate on a fourth optical component mounting structure 160, and coupled into an output port or the endface 124 of the output fiber pigtail 120. The termination of the output fiber is supported above the substrate 116, via a second fiber mounting structure 162.

hi this way, the SOA system is integrated on a common substrate with isolation. This implementation allows for the addition of amplification capabilities in a compact form-factor, which is applicable not only to general amplification applications but also as a subsystem in larger optical systems providing higher levels of functionality.

hi the preferred embodiment, a photodetector is additionally integrated within the SOA system 100. Specifically, a photodetector 126 is installed in the bench 116 to detect the power of the amplified optical signal. Preferably, this signal is used as a feedback control signal to regulate the level of electrical-drive being provided to the semiconductor optical amplifier chip 102.

According to the preferred embodiment, a polarization independent scheme is used to detect the strength of the amplified optical signal. Specifically, a reflecting component is inserted into the beam path of the amplified optical signal to reflect a portion of this optical signal to photodetector 126. In the preferred embodiment, a small portion of the cross-section of the amplified optical signal beam is scattered. This has advantages relative to half mirrors, for example, that are installed across the entire beam path since the reflectivity of such devices is typically polarization dependent.

the preferred embodiment, a mounting structure 158 is inserted to nick an outer cross-sectional portion of beam of the amplified optical signal and thereby scatter a portion of the amplified optical signal to be detected by the photodetector 126. In alternative embodiments, the portion of the amplified optical signal can be specularly reflected to the photodetector 126

According to one manufacturing technique, the optical signal link or path through the system 100 is activated and the mounting structure is placed or deformed into the beam path such that it interrupts less than 5% of the beams' power, and specifically less than 1% in the preferred embodiment. Fig. 2 illustrates a second embodiment of an SOA system 100, which has been constructed according to the principles of the present invention.

Specifically, a wavelength division multiplex (WDM) signal source 10 generates the input optical signal to be amplified. This signal is received by a circulator 30, in one embodiment, which circulator passes the optical signal to the SOA system 100. Alternatively other coupling system can be used.

As described previously, the optical fiber passes into the hermetic package 112 via a fiber feedthrough and is terminated above the optical bench 116. Specifically, the endface is held above the optical bench 116 via a fiber mounting structure 150.

This embodiment is a single physical-fiber port design. Specifically, only a single fiber passes into the module 112. As a result, fiber 118 functions both the input fiber and output fiber. Additionally, the fiber endface functions both as the optical signal input port 122 and the output port 124 for the amplified optical signal.

Specifically, the diverging beam from the fiber endface or input port 122 is collimated by a collimating lens 132. As described previously relative to Fig. IB, the lens is held on an optical component mounting structure 152 on the bench 116.

The optical signal beam is then focused by a focusing lens 134 (held on a optical component mounting structure 154) onto the semiconductor optical amplifier chip 102.

The optical signal is amplified in the chip. The partially amplified optical signal having made one pass through the chip is then reflected to pass through the chip 102 a second time. This double pass arrangement can be accomplished by coating the back facet B of the chip 102 with a high reflectivity thin film coating. In an alternative embodiment, a discrete reflector is located behind the back facet of chip 102. This reflects the light to re- enter the chip 102. In one implementation of this discrete reflector configuration, the reflector 144 has a concave shape to refocus the beam onto the back facet B of the chip 102. In alternative embodiments, additional focusing optics can be installed in the beam path between the back facet B and the reflecting structure 144. The fully amplified optical signal is emitted from the front facet F of chip 102 on the second pass. It is emitted as a diverging beam and is collimated by the focusing lens 134. The amplified optical signal passes from the focusing lens 134 to the collimation lens 132, which now functions as a focusing lens to couple the amplified optical signal into the fiber 118 by focusing it onto the endface 122/124. The amplified optical signal now passes through the fiber 118 now functioning as the output fiber to circulator 130 to be directed to the WDM photodetector 20.

The embodiment of Fig. 2 has provisions for detecting the amplitude of both of the input optical signal and the amplified optical signal. Specifically, an input photodetector 142 detects the level of the input optical signal. Output photodetector 126 detects the level of the amplified optical signal.

Specifically, reflective structures 158, 164 are inserted into the beam paths of both the input optical signal and the amplified optical signal. Specifically, structure 164 specularly reflects or scatters the input optical signal to be detected by photodetector 142. Structure 158 specularly reflects or scatters light to be detected by the output signal detector 126. As a result, the second embodiment is capable of modulating the level by the chip 102 is energized based upon and in response to both the level of the input optical signal and the level of the amplified optical signal.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. For example, in some embodiments, one or both of the isolators 128, 130 are not integrated within the package 112. In this configuration, separate pigtailed isolators are used. Further, single lens systems are used between each endface 122, 124 and the chip 102 in some configurations.

Claims

CLAIMSWhat is claimed is:

1. A semiconductor optical amplifier system, comprising: a package (104); an optical bench (116) within the package (104); a first fiber pigtail (118) entering the package (104) and connecting to the bench

(116) to supply an input optical signal; a second fiber pigtail (120) entering the package (104) and connecting to the bench (116) to receive an amplified optical signal; and a semiconductor optical amplifier chip (102), connected to the bench, that converts the input optical signal to the amplified optical signal.

2. A semiconductor optical amplifier system as claimed in claim 1, further comprising an isolator (128) for suppressing back reflections into the first fiber pigtail (118).

3. A semiconductor optical amplifier system as claimed in any of the preceding claims, further comprising an isolator (130) for suppressing back reflections into the semiconductor optical amplifier chip (102).

4. A semiconductor optical amplifier system as claimed in claim 1, further comprising: a first isolator (128) installed on the bench (116) between a termination (122) of the first fiber pigtail (118) and the semiconductor optical amplifier chip (102) for suppressing back-reflections; a first lens (132) installed on the bench (116) between the first isolator (128) and the termination (122) of the first fiber pigtail (118) for guiding light emitted from the termination (122) of the first fiber pigtail (118); a second isolator (130) installed on the bench (116) between a termination (124) of the second fiber pigtail (120) and the semiconductor optical amplifier chip (102) for suppressing back-reflections; and a second lens (138) installed on the bench for guiding light exiting from the second isolator (130) into the second fiber pigtail (120).

5. A semiconductor optical amplifier system as claimed in any of the preceding claims, further comprising a fiber mounting structure for attaching the termination (122) of the first fiber pigtail (118) to the bench (116).

6. A semiconductor optical amplifier system as claimed in any of the preceding claims, further comprising a fiber mounting structure for attaching the termination (124) of the second fiber pigtail (120) to the bench (116).

7. A semiconductor optical amplifier system as claimed in claim 4, further comprising:

a third lens (134) installed on the bench (116) between the first isolator (128) and the semiconductor optical amplifier chip (102) for coupling light being emitted from the first isolator (128) into the semiconductor optical amplifier chip (102); and

a fourth lens (136) installed on the bench (116) between the second isolator (130) and the semiconductor optical amplifier chip (102) for coupling light being emitted from the semiconductor optical amplifier chip (102) into the second isolator (130).

8. A semiconductor optical amplifier system as claimed in any of the preceding claims, further comprising a monitoring diode (126) that receives a portion of the amplified optical signal

9. A semiconductor optical amplifier system as claimed in claim 8, wherein the portion of the amplified optical signal is from only part of a cross-section of a beam emitted from the semiconductor optical amplifier chip (102).

10. A semiconductor optical amplifier system as claimed in claims 8 or 9, further comprising a structure (158) positioned at an edge of the beam to direct the portion of the beam to monitoring diode (126).

11. A semiconductor optical amplifier system, comprising: a hermetic package (104); an optical bench (116) within the package (104); a fiber pigtail (118) entering the package ( 104) and connecting to the bench (116) to supply an input optical signal and receive an amplified output optical signal; and a semiconductor optical amplifier chip (102) connected to the bench (116) that converts the input optical signal to the amplified optical signal.

12. A semiconductor optical amplifier system as claimed in claim 11, further comprising: a first lens (132) installed on the bench (116) between a termination (122) of the fiber pigtail (118) and the semiconductor optical amplifier chip (102); and a second lens (134) installed between the first lens (132) and the semiconductor optical amplifier chip (102).

13. A semiconductor optical amplifier system as claimed in any of the claim 11 or

12, further comprising a fiber mounting structure (150) for attaching a tennination (122) of the fiber pigtail (118) to the bench (116).

14. A semiconductor optical amplifier system as claimed in any of the claims 11 through 14, further comprising a reflector (144) for double-passing the optical signal through the semiconductor optical amplifier chip (102).