GB2278716A

GB2278716A - Optical fibre laser incorporating polymeric waveguide

Info

Publication number: GB2278716A
Application number: GB9410526A
Authority: GB
Inventors: Hans Kragl; Wolf-Henning Rech
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1993-06-05
Filing date: 1994-05-26
Publication date: 1994-12-07
Anticipated expiration: 2014-05-26
Also published as: GB9410526D0; FR2706090A1; KR950002133A; DE4318752B4; JPH0722673A; GB2278716B; FR2706090B1; DE4318752A1

Description

2278716 1 Resonator device

Prior art

The invention proceeds from a resonator device according to the preamble of the main claim. A laser is already known from "Semiconductor Lasers for Coherent Lightwave Communication" by N. K. Dutta, in SPIE, Vol.1372, Coherent Lightwave Communications (1990), in which a pumped light source as well as a resonator device and a gain medium are integrated in a semiconductor. The resonator device comprises the interface between the semiconductor and air as well as a Bragg reflector, which is structurally incorporated into the semiconductor. The gain medium effects amplification of the light which passes back and forth between the two reflectors. It is further known from "Single-mode-fiber ring dye laser" by W. V. Sorin and M. H. Yu in Optics letters Vol.10 No.11, 1985, to install a ring fibre resonator downstream of a pumped light source. The gain medium is integrated into the ring fibres. Furthermore, from "Polymer Electro-optic DevicesH by G. R. M8h1mann, Akzo Research Laboratories, Arnhem. a Mach-Zehnder interferometer is known, by means of which an optical signal may be amplitude-modulated. The known resonator devices are very expensive to manufacture because of the use of semiconductors. what is more, the generated light has a relatively broad frequency spectrum.

Advantages of the invention In contrast, the resonator device according to the invention having the characterizing features of the main claim has the advantage that the resonator device, owing to the easy 2 processibility of plastic material, is inexpensive to manufacture. A further advantage is that it may easily be realized with relatively large dimensions, thereby resulting in a high frequency selectivity and, at the same time, in low sensitivity to influence by optical reactions. There is also a high probability of single-mode operation given a construction with Bragg reflectors. Furthermore, as a result of the high frequency selectivity of the resonator device, the disturbing frequency shift caused by the useful modulation of the light output power is much less pronounced than, for example, in conventional semiconductor lasers.

By virtue of the measures outlined in the sub-claims, advantageous developments and improvements of the resonator device indicated in the main claim are possible.

Of particular advantage is the construction of the gain medium as part of the optical waveguide as, by said means, a realization of the resonator device plus gain medium which is space-saving and easy to manufacture is achieved. It is further advantageous to couple the gain medium in the form of a doped optical fibre to the reflectors of the resonator device since laser-active doping material for the gain medium, which cannot easily be admixed to the polymer material of the optical waveguide or may be used only in such low concentrations that the resulting large length of the amplification region of the optical waveguide exceeds the dimensions achievable with a polymer in a planar structure, may by means of the doped optical fibre be easily coupled to the resonator device. A heating device in the region of the reflectors is advantageously used for thermooptical variability of the resonant frequency of the resonator device, thereby resulting in a variable design of the laser. Fashioning of the heating device in a wave form is advantageous insofar as an efficient thermal coupling to the reflectors may be achieved while taking up little space. A further advantage arises by providing a Mach-Zehnder structure 3 in the region behind the resonator device, thereby enabl'ing modulation of the laser light. The Mach-Zehnder structure may be manufactured in the same manufacturing process as the resonator device so that its manufacture is easy. The construction of the Mach-Zehnder structure with three electrodes leads to advantageous phase modulation in opposition of the laser light, as a result of which the amplitude modulation effect of the Mach-Zehnder structure is intensified. It is also advantageous to integrate the second reflector in a semiconductor substrate since a combination with further semiconductor components is possible. An advantage further arises when a semiconductor light source with a light guide designed as a gain medium is disposed in the semiconductor substrate and is coupled to the reflector structurally incorporated into the optical waveguide, as a result of which a simple realization of a semiconductor laser with a high frequency selectivity is achievable. It is also advantageous to dispose the optical waveguide and the light guide designed as a gain medium, in which the light beam is guided, in the region of the butt joint between the light guide situated in the semiconductor and the optical waveguide at an angle other than 900 relative to the butt joint, so that lightwaves reflected at the butt joint are-reflected and scattered into a region outside of the optical waveguide and the light guide designed as a gain medium without disturbing the course of the non-reflected lightwaves. By using a polymer substrate, the resonator device according to the invention may moreover advantageously be manufactured in multiples and also with further optical and mechanical components on a substrate in a single manufacturing process.

Drawings Embodiments of the invention are illustrated in the drawings and explained in greater detail in the following description.

The drawings show:

4 Figure 1 a laser with a resonator device, Figure 2 a resonator device with a gain medium in the form of a doped optical fibre, Figure 3 a resonator device with a heating device, Figure 4 a resonator device with a Mach-Zehnder structure, Figure 5 a laser with a semiconductor light source and a resonator device.

Description of the embodiments

Figure 1 shows a laser with a resonator device. The laser has a light source 1 for delivering pumped light 3, to which a glass fibre 2 is connected. The pumped light 3 is guided in said glass fibre. An optical waveguide 22 is integrated in a polymer substrte 4 which serves as the carrier material for the resonator device. The optical waveguide 22 has two Bragg reflectors 5 spaced apart from one another. The part of the optical waveguide 22 lying between the Bragg reflectors 5 is designed as a gain medium 6. The optical waveguide 22 with the Bragg reflectors 5 forms a resonator 9. The resonator device comprises the polymer substrate 4 and the resonator 9 disposed in the polymer substrate 4. The glass fibre 2 is coupled at one end of the optical waveguide 22. At the other end of the resonator 9, there is coupled to the optical waveguide 22 a further glass fibre 7, in which laser light 8 produced by the arrangement comprising light source 1 and polymer substrate 4 plus resonator 9 is guided.

The pumped light 3 emitted by the light source 1 passes through the glass fibre 2 to the resonator 9. The pumped light 3, which has a shorter wavelength than the laser wavelength to be attained, passes through the first Bragg reflector 5 into the region of the optical waveguide 22 designed as a gain medium 6 because the Bragg reflector 5 has a high transmission towards shorter wavelengths than the reflection wavelength. The optical waveguide 22 is made of a polymer, whose refractive index is higher than the refractive index of the polymer substrate 4 and at the same time roughly equal to the refractive index of the glass fibres 2, 7 in order to keep optical losses low. The light, which passes from the light source 1 into the region of the optical waveguide 22 designed as a gain medium 6 and is amplified there by means of the process of stimulated emission, passes from there to the second Bragg reflector 5, where all light of the wavelength not corresponding to the resonance wavelength of the Bragg reflector 5 is reflected. As a result of repeated reflection between the Bragg reflectors 5 under the action of the gain medium 6, amplified coherent light of a low frequency bandwidth passes through the second Bragg reflector 5 into the further glass fibre 7, where it is available as laser light.

A suitable gain medium 6 is, for example, a doped polymer or ormocer, into which rare earths such as erbium, for example, have been incorporated. Otherwise, the gain medium 6 may first be introduced into a suitable carrier, e.g. glass, and said doped material in the form of ultrafine particles may be admixed, with adaptation of its refractive index, to a polymer. Also known are polymers, which themselves possess electroluminescence and so are in principle suitable for use as a gain medium 6.

Figure 2 shows a further embodiment of the resonator device, with the numbering of Figure 1 being retained. Situated in the polymer substrate 4 is the optical waveguide 22 having, integrated therein, the Bragg reflector 5 to whose input the glass fibre 2 is connected. Further provided in the polymer substrate 4 is a further optical waveguide 23, which has the second Bragg reflector 5. The gain medium 6 takes the form of a doped optical fibre 24 and connects the second end of the optical waveguide 22 to one end of the further optical waveguide 23. The second end of the further optical waveguide 23 leads to the further glass fibre 7.

6 Said construction is particularly advantageous in cases where the laseractive material for the gain medium 6 cannot easily be admixed to the polymer material of the optical waveguides 22, 23 or may be used only in such low concentrations that the resulting long length of the amplification region of the optical waveguides 22, 23 exceeds the dimensions achievable on the polymer substrate 4. The mode of operation of the resonator device shown in Figure 2 corresponds to that of the resonator device shown in Figure 1. The light path here extends from the glass fibre 2 via the optical waveguide 22, the optical fibre 24 and the further optical waveguide 23 into the further glass fibre 7.

It is moreover possible to couple the gain medium 6, in the form of a subsance dissolved in a liquid, in an evanescent manner to the optical waveguide 22. Said manner of coupling may likewise be implemented with a doped glass.

Figure 3 shows a resonator device according to the invention with a heating device. In its construction, the resonator device initially corresponds to the resonator device shown in Figure 1. In addition, the resonator device shown in Figure 3 has two connections 11, which are connected to a heating loop 10 arranged in a wave form above the optical waveguide 22.

Current flowing through the heating loop 10 heats the optical waveguide 22 and, in particular, the Bragg reflectors 5. As a result of thermal expansion, on the one hand the refractive index and on the other hand the volume of the optical waveguide 22 varies, so that the thermal action effects a variation in the optical properties of the resonator 9. In said manner, the resonant frequency of the resonator 9 is thermooptically adjustable. The heating loop 10 may, for example, be a vapour-deposited metal layer.

Figure 4 shows a resonator device, whose construction was described in Figure 1, with a downstream Mach-Zehnder 7 structure. The optical waveguide 22 verges, downstream 'Of the resonator 9, into two sub-waveguides 28, 29 which lie substantially parallel in the polymer substrate 4. An electrode 12 is situated above one sub-waveguide 28. A further electrode 27 lies at the side of the other subwaveguide 29 remote from the sub-waveguide 28. Situated between the two electrodes 12 is a third electrode 25 which partially overlaps the other sub-waveguide 29. The electrode 12 and the further electrode 27 are connected in an electrically conductive manner to one another and have an electrode connection 13. The third electrode 25 is provided with a further electrode connection 26. Downstream of the electrodes 12, 25, 27, the two sub-waveguides 28, 29 verge into one another again before opening into the further glass fibre 7. The iub-waveguides 28, 29 are, at least in the region of the electrodes 12, 25, 27, made of an NLO (nonlinear optical) polymer.

An electric field existing between the electrodes 12, 25 and 27, by way of the properties of the non-linear optical polymer, varies the speed and hence the phase angle of the light guided in the sub-waveguides 28, 29. As a result of control in phase opposition of the phase angle of the light in the two sub-waveguides 28, 29 and joining of the light signals phase-modulated in opposition, an amplitude-modulated light signal arises in the optical waveguide 22 downstream of the junction and hence in the further glass fibre 7.

Figure 5 shows a laser with a semiconductor light source and a resonator device. The semiconductor light source 17 comprises a semiconductor substrate 14, in which is disposed a light guide 15 designed as a gain medium 6. Situated above the light guide 15 is an injection electrode 16 with an injection electrode connection 21. A boundary surface of the semiconductor substrate 14 forms a butt joint 19 with the polymer substrate 4. The light guide 15 continues in the optical waveguide 22, which lies in the polymer substrate 4.

8 The optical waveguide 22 contains the Bragg reflector 5 and verges into the further glass fibre 7. The light guide 15 for a large part of its length extends in a straight line and then changes from the position at right angles, to one at a slightly acute angle (e.g. 800) to the butt joint 19. The optical waveguide 22 starts at the same acute angle to the butt joint 19 before then likewise changing back into a position at right angles to the butt joint 19.

Electrons injected via the injection electrode 16 lead to a stimulated emission of electrons in the light guide 15 designed as gain medium 6. At the interface opposite the butt joint 19 between the semiconductor substrate 14 and the medium of a lower refractive index surrounding it, which interface forms an interiace reflector 18, the emitted lightwaves are then reflected towards the butt joint 19 where they pass into the optical waveguide 22. The Bragg reflector 5 provided in the optical waveguide 22 likewise effects a reflection. By virtue of the optical waveguides 15, 22 being at an acute angle to the butt joint 19, reflections at the butt joint 19 are scattered into the substrates 14, 4 and so do not disturb the transmitted lightwaves of coherent light.

The resonator device, which contains the polymer substrate 4, may be covered by a lid so as to be protected from environmental influences. Said lid is advantageously also made of a polymer, which may preferably be the same polymer as the polymer of the polymer substrate 4. Connection of the lid to the polymer substrate 4 may be effected using a polymer adhesive. Provided the polymer adhesive is suitable as a gain medium 6, an already preformed negative structure in the lid for two Bragg reflectors 5 may be provided, and the optical waveguide 22 with the Bragg reflectors 5 may be formed by the polymer adhesive during the assembly process of the lid onto the polymer substrate 4.

9

Claims

Claims

Resonator device ' for generating coherent light of a low frequency bandwidth, wherein pumped light is supplied to the resonator device, having two reflectors, in particular Bragg reflectors, between which a gain medium is disposed, characterized in that at least one reflector (5) takes the form of a reflector (5) structurally incorporated into an optical waveguide (22) made of a polymer wich is coupled to the gain medium (6).
2. Resonator device according to claim 1, characterized in that the gain medium (6) and the optical waveguide (22) are of an integral construction.
3. Resonator device according to claim 1, characterized in that the gain medium (6) takes the form of a doped optical fibre (24) which is coupled by at least one end to the optical waveguide (22).
4.

Resonator device according to one of claims 1 to 3, characterized in that the optical waveguide (22) in the region of at least one reflector (5) has a heating device, by means of which the resonant frequency of the resonator device is thermooptically variable.
5. Resonator device according to claim 4, characterized in that the heating device has a wave-form heating loop (10).
6. Resonator device according to one of claims 1 to 5, characterized in that disposed in the region downstream t of the resonator device is a Mach-Zehnder structure, by means of which the coherent light may be electrooptically modulated.
Resonator device according to claim 6, characterized in that the MachZehnder structure has at least three electrodes (12) which are used for phase modulation in opposition of the coherent light.
8. Resonator device according to one of claims 1 to 7, characterized in that the second reflector (5) is integrated in a semiconductor substrate (14).
9. Resonator device according to claim 8, characterized i that integrated in the semiconductor substrate (14) is a semiconductor light source (17), which generates the pumped light (3) and has a light guide (15) designed as a gain medium (6), which is disposed in the semiconductor substrate (14) and on one end of which the optical waveguide (22) with the reflector (5) is disposed.
10. Resonator device according to claim 9, characterized in that the optical waveguide (22) via a butt joint (19) forms the continuation of the light guide (15) designed as a gain medium (6) and that the longitudinal axes of the optical waveguide (22) and of the light guide (15) form with the butt joint (19) between semiconductor substrate (14) and optical waveguide (22) an angle other than 900.
11. Resonator device according to one of claims I to 10, characterized in that the optical waveguide (22) is disposed in a polymer substrate (4).
12. Either of the resonator devices substantially as herein described with reference to the accompanying drawings.