WO1995013639A1

WO1995013639A1 - Semiconductor lasers

Info

Publication number: WO1995013639A1
Application number: PCT/GB1994/002502
Authority: WO
Inventors: Jayanta Sarma; Ian Middlemast; Ian Hugh White; Richard Vincent Penty; Frances Ruth Laughton; Keith Alan Shore; Trevor Ryan
Original assignee: British Technology Group Limited
Priority date: 1993-11-12
Filing date: 1994-11-14
Publication date: 1995-05-18
Also published as: GB9323388D0

Abstract

A semiconductor laser has an optical cavity formed between a pair of reflecting surfaces and a region or regions having a non-uniform cross section with a minimum value at a predetermined location between the reflecting surfaces so that the optical energy at both surfaces is distributed over a broader area and hence the optical intensities at the facets are reduced. The tapered bow-tie structures have a sufficiently gradual change to ensure adiabatic spatial mode-shape variation permitting single spatial mode operation. The laser is also used for Q-switching or mode-locking.

Description

Semiconductor Lasers

This invention relates to lasers and, in particular, to lasers fabricated from semiconductor materials.

High power lasers now find an enormous range of applications including optical machining, medical treatments, chemical and biological analysis, materials preparation, and free space inter-satellite communications. Whilst solid state and gas lasers have proved for some time to be capable of producing such high powers, they have major disadvantages in being relatively inefficient and cumbersome. As a result there has been considerable interest in developing compact efficient lasers able to meet the application requirements. Recently the development of semiconductor laser diode pumped solid state lasers have had major impact in replacing the large frame lasers for a wide range of applications. However, significantly greater advantages would be achieved if tiny single chip semiconductor diode lasers were developed to meet these applications.

A similar argument also can be made for the development of single chip high power picosecond pulse laser sources. Here, in recent years, a large range of applications have arisen for optical laser sources able to generate high power pulses with temporal widths of lOOps or less. These applications cover a wide range of fields including for example spectroscopy, sensing, LIDAR, measurement of ultrafast biological and chemical phenomena, and pumping of nonlinear optical systems for frequency doubling and optical parametric oscillators. Here again, many of the existing applications require higher peak optical powers than those which diode lasers have been able to generate to date. The advantages of developing diode lasers for such applications, however not only includes compactness and efficiency, but also stability and pulse quality, as existing large frame lasers require precise cavity alignment which is susceptible to mechanical movement and vibration. Single chip laser systems alternatively require no external alignment and hence can be very stable.

The main problems that occur in using semiconductor lasers for high power output are saturation, and catastrophic optical damage due to optically induced heating at the facets. Both these effects occur in typical semiconductor lasers due to the small lateral dimensions involved. Nevertheless, various improved designs combined with careful fabrication have been carried out to enable quite high powers (around 500mW) to be generated from narrow, single stripe semiconductor lasers. However, for very much higher powers (of, say, several watts) even the above modifications are not adequate and methods for achieving increased power, such as increasing the lateral dimension of the device to keep the power density small so that the problems of saturation and catastrophic optical damage are avoided. To this end broad stripe lasers and laser arrays have been designed and operated but these have been beset with another problem, namely that of lateral multi- moding leading to undesirable far-field patterns.

In order to attain high power along with a narrow beam shape, the recent trend in device development has been to combine broad area with some form of mode filter. To date this is most economically achieved by using a slowly tapered device structure. The popular approach has been to use the tapered section as a power amplifier with the narrow end of the tapered amplifier fed from a matching, conventional narrow stripe (single lateral mode) laser. Integrated laser-amplifier structures have also been developed. The narrow end of the tapered amplifier is designed to support only a single lateral mode and the slow (adiabatic) taper in these devices then ensures that even at the broad end, which can support multiple modes, the power is retained predominantly in the fundamental mode so that the desired narrow far-field pattern is produced.

For some applications, such as the pumping of fibre amplifiers or solid state lasers, the sophistication of a master oscillator power amplifier configuration is not needed and a simpler high power source but with a narrow far-field is sufficient; this may be obtained if the tapered device is operated as a laser. In the power amplifier the light travels only in one direction as it gets amplified - from the narrow, input end where the power is small to the broad, output end where the total power is large, but the power density still remains small due to the increased width. However, with the tapered device operated as a laser, the single-pass condition is no longer valid and both forward and reverse propagating light beams exist in the device with the consequence that very high optical power densities occur at the narrow end of the tapered structure. Hence the problems of saturation and. particularly, catastrophic optical damage are greatly accentuated. Nevertheless, several watts of output power with a narrow far-field have been reported, from such tapered laser structures. To reduce the problems of catastrophic optical damage this device has moderate anti-reflection coating on the broad facet and also relies on the diverging reflected light from this facet to produce a reduced optical intensity in the narrow region. However, this arrangement must make the device less efficient thereby requiring higher injection currents for satisfactory operation. The use of higher current adds to the temperature increase in the semiconductor and this, in turn, leads to a further decrease in efficiency. It is, therefore, very important to investigate schemes that can be implemented simply, which reduce the limitations posed by catastrophic optical damage, and improve device efficiency while retaining the desired device characteristics.

In terms of picosecond pulse sources, similar limits are found, in that high power picosecond pulse generation typically causes strong depletion of carrier concentration within the laser. This inhibits further optical amplification. As a result, the power obtained from Q-switched laser diode sources is found to be limited by the active volume of the laser. However large area lasers are susceptible to multimode operation. In terms of mode-locked pulse generation, further difficulties are encountered as reduced volume devices are susceptible to high photon and carrier induced chirp, causing reduced picosecond pulse performance. According to the present invention there is provided a semiconductor laser formed in a body of semiconductor material having an optical cavity formed between a pair of at least partially reflecting surfaces and a region or regions of said body having non-unfoπn electrical characteristics wherein the cross-sectional area of said cavity between said surfaces is non-uniform and has a minimum value at a predetermined location between said pair of partially reflecting surfaces.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of a double-ended tapered (bow-tie ) laser diode in accoradance with a specific embodiment of the invention: Figures 2 to 5 are schematic plan views of alternative embodiments;

Figure 8 shows dimensions of a typical device Figures 6, 7 to 9 are traces representing measurements made on lasers in accordance with specific embodiments, and Figures 10 to 17 are schematic plan views of yet further embodiments of the invention.

Referring now to the drawings, Figure 1 is a diagrammatic representation of a double-ended tapered (bow-tie laser diode which has a narrow optically active region ( 1 1 located towards the centre of the device. This narrow region is well removed from the two optically-reflecting facets (2) and (3). From the central narrow region (1) the device structure tapers to increase the volume of the optically active region to reach adjacent the facets (2)(3). Locating the narrow central region away from the facets very significantly reduces the limitations posed by catastrophic optical damage while at the same time providing a simple but effective means of spatial mode filtering. The facets may be provided with anti-reflection and high reflection coatings to allow optimum output power operation. Preferably, also these are non-absorbing mirrors to allow the highest possible output powers without catastrophic optical damage. In a preferred embodiment the semiconductor material used is multi-layer, double heterostructure GaAs/GaAlAs layer system (4a - 4e). The change in the volume of the active region with longitudinal position is achieved by having an injection current contact (5), wich varies in (lateral) width (6) being narrowest towards the centre of the device and increasing towards the facets (2)(3). Alternatively or additionally, the (transverse) thickness of the active layer (6) of the double-heterostructure semiconductor layers may be varied. Ideally, this varying profile is so shaped as to provide an adiabatic transfer of optical energy between the facets.

Still further embodiments expand the optical field towards the facets by means of refractive index profiling. The present invention also contemplates the use of plane, angled and curved facets (2)(3) to achieve desired operational characteristics of the double-ended tapered device. With complementary curved facets a larger fraction of the optical power reflected from the facets remains within the optically active tapered volume to improve device efficiency.

Segmentation of the current contact (5) to enable easy longitudinal and or lateral injection current profiling forms another feature of this invention. The invention also includes non- identical injection current profile (5) and optical waveguide structure (8) geometries. (Figure 4) although the general shapes of both will be that of a double-ended taper.

The shape of the taper is referably chosen to ensure that the optical energy is predominantly maintained in the fundamental (lateral) mode. Periodic and/or aperiodic modulation of the taper width, (Figure 5). or of the other dimensions, along the length provide a structure which leads to a strong, stable control of the lasing wavelength and thereby to a narrow line-width operation. The range of structural geometry and other parameters (as described) that may be varied in the design of the bow-tie laser enable it to have the potential for very high power optical signal generation together with narrow beam and spectral quality. The use of cascaded laser drive sections to minimise gain saturation effects, the use of controlled multiple guide structures to ensure single lateral/transverse mode operation, and the use of longitudinal features give a range of methods to optimise device performance characteristics.

Such tapered structures have considerable potential for ultra-high power picosecond pulse generation by Q-switching and mode-locking. Figure 3 shows a schematic diagram of a bow-tie laser, but with triple contacts, for Q-switching operation. By biasing the central contact so that it acts as a saturable absorber, Q-switching is achieved in the conventional manner when the end contacts are driven with a large forward bias. The saturable absorber is chosen in the centre as the absorber acts to reduce the optical power density in a region which would otherwise have very large power densities. The use of distributed saturable absorbers and alternative waveguide structures are also lgives improved performance.

In a further embodiment, a triple contact bow-tie laser structure is used to generate very high power picosecond pulse generation by Q-switching. By using tapered waveguides, the device retains well defined near and far field distributions, and hence is suitable for many applications requiring beam control. The device is directly compared with a narrow stripe laser of similar structure.

A bow-tie laser in accordance with one specific embodment of the invention has an 80 micron long central waveguide section of width 10 microns, and two tapered end guides which are both 200 microns long and splay out to a width of 45 microns at the device facets (Figure 8). The rib waveguides are formed by ion beam etching and the device contacts are segmented to allow each section to be independently driven. The devices have conventional GaAs/GaAl As bulk active layer configurations. Without facet coatings, they operate with a threshold current of 180mA.

Under a conventional Q-switched operation, the end sections are pumped together at around five times threshold with a pulsed electrical drive (HP214B generator). Electrical pulses of 1-10ns duration at a 1MHz repetition rate are generated by shaping the electrical train with step recovery diodes and are impedance matched at the laser chip with a 50Q

SUBSTITUTE SHEET (RULE 26) resistor. A reverse DC bias to the central absorber section allows pulse profile and peak power to be controlled. It can also be used to eliminate pulse tails, which are undesirable for a number of applications.

The pulse profile is monitored using an 18GHz InGaAs p-i-n photodiode with a 12GHz bandwidth HP54120 sampling oscilloscope. The pulse width is determined by autocorrelation. Peak powers and pulse energies are calculated from average power measurements.

As single spatial moded operation is desirable for efficient fibre coupling, the single moded nature of the near field has been confirmed with high resolution under Q-switched operation. The optical spectrum has been assessed with a 1200 line/mm grating and infra¬ red camera configuration with 0.7 nm resolution.

Picosecond pulses with energies of 50pJ are achieved by applying 50V peak. 5ns electrical pulses to the end tapered sections while a 3V reverse bias is applied to the central absorber. The resolution limited oscilloscope trace given in Figure 9 shows the asymmetric pulse generated. The autocorrelation trace in Figure 10 indicates a single sided exponential pulse of 8ps full width at half maximum (FWHM) pulse width. With negligible pulse tails, peak power levels of 6.2 W are measured for the Q-switched bow-tie laser.

Variation in the electrical drive through an increase in electrical pulse length and absorber bias allows multiple pulsing at repetition rates as low as 1GHz. No change in optical peak power is observed for increases in either pulse length above 5ns or for voltages above 50V in agreement with the principles of Q-switching. Spectral assessment indicates a spectral width of 1.35nm (FWHM). The optical spectrum is given in Figure 4. This indicates a small number of chirped modes under Q-switched operation.

In order to assess the improvement achieved by using the tapered bow-tie structure, a non-tapered control device of similar length from the same wafer has been Q-switched in a similar manner. This device consists of a waveguide of width 5 microns, the p-side electrical contact being split longitudinally into three sections of 200, 40 and 200 microns length. Using this device, self Q-switched pulses with sech² profiles of 35ps FWHM width and 7pJ energy have been achieved for 25V gain bias and - 7V absorber bias. As a result, it is found that the bow-tie device achieves a sevenfold increase in pulse energy and thirty-fold increase in peak power over non-tapered devices from the same wafer. Further improvements may be achieved by reduced diode resistance and improved current confinement for more carefully grown wafers.

In terms of mode locking, the bow-tie laser structure has substantial advantages as it increases the waveguide cross-section for regions in which the optical power is likely to be very high. As a result, carrier and photon induced refractive index changes are minimised (as these are primarily functions of intensity rather than optical power) and hence chirp is also minimised. This gives improved mode locking performance.

Two types of Q-switching developed recently offer additional advantages. Firstly, the modified Q-switching of multicontact structures generates high peak power, bandwidth- limited picosecond pulses in distributed feedback lasers. Pulses may be generated on demand with peak optical power increases of up to four times as compared to conventional Q-switching. A second technique has also been developed to overcome the repetition rate and pulse timing jitter limitations from available pulse generators. Ultra-low timing jitter, high energy pulsing at gigahertz repetition rates is achieved by driving the absorber with sinusoidal radio frequency modulation under forced Q-switching. Such techniques can be readily applied to bow-tie structures for significant further performance improvements.

As an alternative to taper structures symmetric about the midpoint in the longitudinal direction, non-symmetric waveguide structures may be employed. Figure 10 shows a possible non-symmetric waveguide structure. Tapers may also be asymmetric about the longitudinal axis.

In yet further embodiments, the taper electrical contact and/or waveguide may be split into two or more parts. Example structures are shown in Figures 11 and 12. By changing the local refractive index by means of thermal, electrical or optical pumping, one or more of the sections may then be used as phase shifters. This gives coupled cavity effects, which may be used to achieve phase or amplitude modulation. An example of an application for this is wavelength tuning, and one specific example of a wavelength-tunable bow-tie laser is shown in Figure 13. The electrical contact and/or the optical waveguide in the tapered region is segmented laterally, so that the injection current densities into sections A and B were not identical. The optical spectrum of the bow-tie laser may thus be made to be single-moded. Furthermore, by altering the current into one or more of the laser sections, the emission wavelength of the bow-tie laser may be altered due to the chanee in the local refractive index. Other effects mav also be used to alter the refracth e index, and hence provide wavelength tuning of the bow-tie laser, for example temperature and optical pumping.

A Maltese cross (turn-stile/four-leaf clover) configuration made of a combination of bow-tie devices or a bow-tie with conventional straight lasers, as shown in Figure 14, may be used for applications such as switching, routing, modulation and mixing.

An additional advantage of the bow-tie, laser is its inherent suitability for use in array form. Bow-tie laser arrays can result in an increase in optical power, and may be phase locked, thus maintaining fundamental mode operation, Two specific examples are shown in Figures 15(a) and (b). Features of the bow-tie laser array are the narrow sections in the centre of the device and, preferably, adiabatic tapers extending towards the facets. These tapered regions act, as for the single element bow-tie laser, so as to expand the optical mode and hence reduce saturation effects. In an array form, a clear advantage of the Bow-tie laser structure is that the, tapered regions also naturally provide strong parallel optical coupling between the array elements. Bow-tie arrays may be used both for cw and short pulse operation, and may be operated using either a single or multiple electrical contacts. They may also be, used in conjunction with other schemes for phase-locking semiconductor laser arrays, for example as anti-guided Bow-tie laser arrays or within a Talbot cavity.

Multimode interference (MMI) couplers may also be used to form one or more regions of the bow-tie laser. Examples of such lasers are shown in Figures 16(a) and (b). The width of peripheral regions 21 and 22 are such that the laser supports only a single lateral mode at these points. Many optical modes may be supported in the central region 23 of the MMI coupler, but by careful design of its dimensions, the imaging properties of the coupler may be used to ensure that the bow-tie laser operates in a single lateral mode. while the central region 23 may be of any width, even to the extent of being wider than the width of the tapered regions at the facets. This further reduces saturation effects in the centre of the device. The incorporation of MMI coupler regions into the Bow-tie laser design can also result in single longitudinal mode operation. The MMI coupler regions may be fabricated in a number of ways, including, for example, deep etching of the semiconductor material and strip-loading.

Techniques such as selective area epitaxy or quantum well disordering may be used to vary the band-gap of the semiconductor material within the laser structure. Example

-δ- reasons for this would be (i) to increase the loss in the centre section, thus enhancing mode- locking and Q-switching operation and (ii) to enhance optical nonlinearities within the laser structure, e.g. second harmonic generation.

The central narrow region and the tapered regions are defined by a combination of the injection current and the processing of the semiconductor materials involved to form a lateral refractive index profile, in order to confine the optical mode, In general, the manner in which one region of the bow-tie laser is defined may differ from that in which another section is defined. This means that, in addition to longitudinal and lateral current profiling, the refractive index profile used to form the optical waveguide need not be constant along the device. As an example, the central region may be formed as for a ridge waveguide laser, that is strongly index guided, while the tapered regions may be defined by the injection current alone, so that no etching of the tapered regions is carried out, and the tapered regions are gain guided. The utilisation of ion-beam and /or chemically etched patterns on bow-tie lasers for achieving required refractive index and/or injection current profiles to attain very much enhanced device operational characteristics.

Buried heterostructure (b-h) semiconductor material may be employed to enhance a range of operational characteristics (e.g., achieve lasing wavelength stability) by better defined mode properties.

Other embodiments exhibit enhanced functionality. For example, use of feature- generated large optical fields may induce a range of optically non-linear operations — e.g., the realisation of an integrated, second harmonic generated (surface emitting) source, see Figure 17.

A bow-tie laser may be used for built-in beam scanning by suitably modifying composite device parameters. Bistable operation may achieve a large hysteresis step while retaining the essentially single transverse mode characteristics.

Bow-tie lasers subject to external optical injection can be utilised to effect nearly degenerate multiwave mixing giving rise to phase conjugate optical signals in transmission and reflection modes of operation. Both, nearly degenerate and highly degenerate. multiwave mixing processes can also be utilised for efficient frequency translation over GHz and THz frequency ranges. The efficiency of the multiwave mixing process in these devices mav be increased bv two means. Maximum cavitv enhancement of multi-wave mixing can be achieved by optimisation of the facet reflectivities. Reduced saturation effects in tapered laser structures significantly assist in optical power levels of both frequency-translated optical signals and generated phase conjugate signals.

One of the advantages of a bow-tie laser is that, although optical signals are generated with high powers, the optical filament is concentrated in the central region in a narrow cross-section. This allows ready control of optical power by direct current or voltage modulation of the centre region and hence low power electrical signals can be used to efficiently modulate high power optical ones. For high speed operation, band gap variation (described above) may be used to enhance the direct modulation capability. Reduced j itter has been achieved by driving the centre section of the device with an r.f. electrical signal. As a result, the bow-tie laser is suitable for Q-switched pulse generation using all-known Q-switching techniques. Pulse generation may be enhanced by (i) enlargement of the centre section to increase the saturation power in the absorber and (ii) the use of transverse segmented contacts or reduced index guiding in the gain regions to reduce gain saturation.

A key feature of the bow-tie laser is that low-bias or reverse bias on either centre or end sections can readily activate mode- locking at high optical powers and a further new feature is that this also improves both the spatial and spectral output.

Claims

1. A semiconductor laser formed in a body of semiconductor material having an optical cavity formed between a pair of at least partially reflecting surfaces and a region or regions of said body having non-uniform electrical characteristics wherein the cross-sectional area 5 of said cavity between said surfaces is non-uniform and has a minimum value at a predetermined location between said pair of partially reflecting surfaces.

6. A semiconductor laser as claimed in claim 3 having a longitudinally-segmented injection contact.an optically active region in which the cross-section increases adiabatically from said predetermined region. 10

3. A semiconductor laser as claimed in claim 2 wherein the shape of the optically active region is defined by the shape of the current injecting electrode. and active layer thickness.

4. A semiconductor laser as claimed in claim 2 wherein the shape of the optically active region is defined by the active layer thickness. 15.

5. A semiconductor laser as claimed in claim 1 having an extended region of uniform cross-sectional area in the vicinity of said predetermined location.

6. A semiconductor laser as claimed in claim 3 having a longitudinally-segmented injection contact.

7. A semiconductor laser as claimed in claim 3 having means for lateral current profiling. 20

8. A semiconductor laser as claimed in claim 3 curved facets to reflect an increased fraction of the optical power into said tapered cavity.

9. A semiconductor laser as claimed in claim 1 having curved facets to shape the profile of the output beam.

10. A semiconductor laser as claimed in claim 1 having a region of the optical cavity 25 operable unenergised to act as a saturable absorber.

11. A semiconductor laser as claimed in claim 1 having a plurality of contacts which operable either as gain or saturable absorber regions to permit optimised Q-switched operation.

12. A semiconductor laser as claimed in claim 1 having a region of optical nonlinearin 30 to achieve mode locking within the optical cavity.

13. A semiconductor laser as claimed in claim 1 having a region of optical nonlinearity to achieve colliding pulse mode locking.

14. A semiconductor laser as claimed in claim 1 having a plurality of optical cavity regions.

15. A semiconductor laser as claimed in claim 1 including means to provide distributed feedback within said optical cavity.

16. A semiconductor laser as claimed in claim 2 having facets provided with at least one anti-reflection coating.

17. A semiconductor laser as claimed in claim 2 having facets provided with at least one high-reflection coating.

18. A semiconductor laser as claimed in either claim 16 or claim 17 wherein the coating on said facets is non-absorbing mirrors to inhibi catastrophic optical damage during transmission of radiation.

19. A semiconductor laser as claimed in claim 1 including a multimode interference coupler.

20. A semiconductor laser as claimed in claim 19 including a multimode interference coupler adapted to operates in a single lateral mode.

21. A semiconductor laser as claimed in claim 1 including a region having a longitudinally varying bandgap.