Monolithic hybrid modelocked 1.3 μm semiconductor lasersP. A. Morton, J. E. Bowers, L. A. Koszi, M. Soler, J. Lopata, and D. P. Wilt Citation: Applied Physics Letters 56, 111 (1990); doi: 10.1063/1.103046 View online: http://dx.doi.org/10.1063/1.103046 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/56/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Active mode locking at 50 GHz repetition frequency by halffrequency modulation of monolithic semiconductorlasers integrated with electroabsorption modulators Appl. Phys. Lett. 69, 2626 (1996); 10.1063/1.117556 Integration of 1.3 μm wavelength lasers and optical amplifiers Appl. Phys. Lett. 57, 1375 (1990); 10.1063/1.103440 Threshold current reduction in patterned quantum well semiconductor lasers grown by molecular beam epitaxy Appl. Phys. Lett. 56, 1825 (1990); 10.1063/1.103196 Highly efficient pseudomorphic InGaAs/GaAs/AlGaAs single quantum well lasers for monolithic integration Appl. Phys. Lett. 56, 1731 (1990); 10.1063/1.103106 Optimum coupling junction and cavity lengths for coupledcavity semiconductor lasers J. Appl. Phys. 57, 740 (1985); 10.1063/1.334721

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Monolithic hybrid mode~locked 1.3 I-tm semiconductor lasers P. A. Morton and J. E. Bowers Department of Electrical and Computer Engineering. University of California. Santa Barbara, California 93106

L A. Koszi, M. Soler, J. Lopata, and D. P. Wilt AT&T Bell Laboratories, Murray Hill, New Jersey 07974

(Received 14 August 1989; accepted for publication 30 October 1989)

We describe the first results of hybrid mode locking combining both active and passive mode locking of a semiconductor laser. These functions are integrated into a monolithic device with a 1.3 11m GalnAsP gain region, an active waveguide, and a saturable absorber. The devices have low threshold currents, and exhibit hysteresis in their light/current characteristics. The long integrated waveguides allmv mode locking at a repetition rate of 15 GHz without the need for an external cavity. Pulse widths as short as 1.4 ps have been demonstrated using the combined effects of active and passive mode locking.

Mode-locked semiconductor lasers are important in high-speed optical communication sytems for time division multiplexing and for high bit rate systems using an external modulator. They are ideal candidates for use in practical commercial electro-optic sampling systems because of their small size, low cost, low noise, and small timing jitter in comparison to the use of more complex and less reliable sources such as pulse compressed Y AG lasers. The modc­locked pulse train can also be used as a low jitter clock signal for high-speed electronic circuits and the short pulse length can be utilized for very high resolution optical time domain. refiectometers. Mode-locked semiconductor lasers have produced ultrashort pulses',2; however, a stable external cavity and a high coupling efficiency is required which poses stability problems when using bulk optics. A monolithic structure overcomes the need for an external cavity by the use of along integrated waveguide. The integrated structure described here is constructed from a single continuous wave­guide so that multiple pulses do not build up from internal reflections. Such multiple reflections have been present in all subpicosecond mode-locked semiconductor laser results to date, and are due to the finite reflectivity of the antireflec­tion-coated laser facet.' In.itial attempts to mode lock a sin­gle long laser device4 resulted in sinusoidal modulation with well below 100% modulation depth. Recently, Tucker et aJ. 5

described results using a laser with an integrated passive waveguide, which led to pulse widths of 4 ps at a 40 GHz repetition rate. This letter describes devices in which, for the first time, we have integrated a gain region with an active waveguide and saturable absorber to provide pulse shorten­ing by both active and passive mode locking.

The monolithic mode-locked lasers6 were fabricated from semi-insulating, channeled substrate, buried hetero­structure 1.3 f.lm GalnAsP laser materia1.7 Each device was fabricated from a single, continuous optical waveguide split into three functionally separate regions by separate contact pads. A schematic diagram of an array of six of these devices is shown in Fig. 1, together with a photograph of such an array. The device consists of a gain section which is 75 11m in length, an active waveguide of 2300 11m, and a saturable absorber of 75 11m. Each section is separated by a 50 pm

spacer region which typically provides 5 kH of isolation between contacts. The threshold current of these devices is typically between 70 and 80 rnA and is uniform across an array of devices. This low value of threshold current for such a long device can be attributed to an extremely high quality optical waveguide with low scattering losses.

The curves for light output versus current in the active waveguide are shown in Fig. 2 for various values of current in the gain section. The saturable absorber section is left un­connected to give the required characteristics; that is, it pro­vides a loss until it is optically pumped up to transparency and then the loss falls to a very small value. When the gain section is left open circuited it acts as another saturable ab­sorber. In this case the threshold current is 72 mA, and the differential quantum efficiency just above threshold is low, due to absorption in the two saturable absorber sections. Both absorber sections become transparent at 85 rnA when a kink occurs in the L-J curve and the differential quantum efficiency then rises to a much larger value, As the current into the gain section is increased, the kink occurs at lower waveguide currents and the differential quantum efficiency increases. The optical output rises dramatically for the small changes in gain section bias which are shown in Fig. 2, as compared to the change in optical output for similar in­creases in the waveguide bias. This occurs because the opti­cal loss of the end segment varies more for a small change in current than the change in gain in the waveguide section. This phenomenon only occurs for small current biases in the gain section, until the point at which this bias alone raises the section to transparency. This phenomenon is similar to the gain lever mechanism described in Ref. 8. By biasing the device with a small or zero current flowing in the gain sec­tion it is possible to use this mechanism to provide more efficient modulation of the device. When the gain section is short circuited and the active waveguide is pumped above threshold, absorption occurs, and a reverse current flows through the gain section, as in a photodetector. This absorp­tion is not saturated as the output power increases, and the differential quantum efficiency stays low up to high current levels.

All of the curves in Fig. 2 show hysteresis above thresh-

111 Appl. Phys. Lett. 56 (2), 8 january 1990 0003-6951/90/020111-03$02.00 (.c) 1990 American Institute of Physics 111

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50 llm

75/lm

)-1

-~f ., .J

h i-I

tJ i-l

h I-'

n i-.J.

2300 !lm ~ -- 75/lm r L

r L.

r L

r II..

'n i..

old, which is the characteristic of a device including a :;mtura­ble absorber.9 The amount of hysteresis is small, which is expected because of the small proportion of the overall loss which is made up by the saturable absorber section (the ac­tive waveguide section is over 20 times longer than the ab­sorber section).

Arrays of six devices are mounted on a BeO substrate for improved heat sinking. This substrate is then mounted onto a microwave fixture to allow high-speed connections to be made to both the gain section and the active waveguide of each of the devices. The microwave fixture incorporates two alumina substrates with microstrip lines in order to make the necessary connections and provide a fan out to 40 GHz K connectors mounted around the side of the fixture. The con­nections from the microstrip to the laser sections are made using gold mesh in order to keep the parasitic inductance small.

The device is biased with a de current to the waveguide and a dc current plus rf modulation is applied to the gain section. The output is taken from the saturable absorber end of the device. This is expected to be smaller than the output from the gain section end, but it is necessary to compromise the caHeeted power in order to provide the shortest possible electrical connection to the gain section to allow modulation at 15 GHz. The optical output is collimated with an objective lens, and is then focused onto a high-speed 36 GHz band-

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0 20 40 60 80 130 120

Waveguide Current (mA)

FIG. 2. Light output vs current in the active waveguide ,cction for various gain section bias levels. The absorber section is open circuited.

112 Appl. Phys. Lett., Vol. 56, No.2, 8 January 1990

FIG. 1. Schematic diagram of an array of monolithic mode-locked devices with an inset showing a photograph of one such array.

width GalnAsp-i-n detector. 10 The output from the detector is taken either to a spectrum analyzer (0-22 GHz) or a 20 GHz sampling oscilloscope.

Small-signal microwave measurements of the frequency response ofthese devices taken on a HP851 0 vector network analyzer show a very large peak at 15 GHz which is the reciprocal of the round trip time of the device. This peak at 15 G Hz was measured to be only 12 dB below the response at SO MHz, while away from the peak the response falls by over 27 dB. This indicates a strong feedback from the integrated cavity, a characteristic of active waveguides,

Under dc bias conditions, the microwave spectrum shows a noise peak at the reciprocal of the overall cavity mund trip time. This noise peak is shown in Fig. 3, together with a trace in which + 10 dBm modulation is applied to the device at the frequency at which the noise trace peaks. The linewidth of the noise peak is around 10 MHz, compared to an instrument-limited Hnewidth measured for the peak un­der modulation of below 30 Hz, The noise peak is suppressed by the presence of the strong modulation signal so that the overall noise level becomes flat for high levels of rf power.

A time domain measurement of the optical output taken on a sampling oscilloscope is shown in Fig, 4 (a). This trace shows a 15 GHz sinusoidal waveform of4 mV peak-to-peak value. The next highest harmonic of the mode-locked pulse train is the third harmonic at 45 GHz, which is well above

·40

FrO. ]. Microwave noise spectrum around the cavity frequency (15 GHz) for (a) no modulation and (b) + IO dBm modulation.

Morton eta!. 112

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(a) Time (ps)

(h) Delay (pS)

FIG. 4. (a) Time domain measurement on a sampling oscilloscope and (b) the second harmollic trace for t 12 dEm modulation at 15. I GHz.

the operating bandwidth of the sampling oscilloscope (20 GHz). A noncollinear second~order intensity autocorrela­tion technique is employed in order to measure the actual width of the optical pulses. Figure 4(b) shows a second­harmonic intensity trace versus the time delay in one "rm of the measurement. This trace shows a full width at half maxi­mum of2.17 ps, which corresponds to a pulse width of 1.4 ps for a sech2 pulse shape. We believe that this is the shortest measurement of a single pulse produced directly from a mode~locked semiconductor laser device. The average out­put power from the device is typically 0.5 mW, giving peak powers of about 20 mW. The optimum biasing conditions necessary to achieve short pulses were a zero bias current in the gain section, and a bias current well above transparency (110 rnA) in the waveguide section. In some devices a low value of isolation between the gain and waveguide section gave rise to different measured values of bias current, al­though the actual amounts flowing through the sections would be close to those described for optimum biasing. In devices with low isolation, the shortest pulses were achieved.

1 i 3 Appl. Phys. Lett., Vol. 56, No.2, 8 .January 1990

This may be due to a traveling-wave effect of the rf modula­tion in which the optical pulse is also partly shaped while passing under the active waveguide. In this case, lower val­ues of rf power ( + 12 dBm) were necessary to produce short pulses than for devices with high values of isolation. Results for devices with high isolation between the wave­guide and gain section showed pulse widths down to 3.5 ps for + 27 dRm applied rf power.

In conclusion, these are the first results for mode lock­ing of a monolithic laser device including a gain section, an active waveguide, and a saturable absorber section. This hy~ hrid mode locking technique includes the effects of both ac­tive and passive mode locking. Monolithic devices provide advantages over the conventional mode locking arrange­ment as a bulk optic external cavity is not required and a single-pulse output is produced. The monolithic devices de­scribed in this letter have produced single optical pulse out­puts with pulse widths as short as 1.4 ps.

This work was supported by a National Science Foun­dation Presidential Young Investigator Award and Tek­tronix. The authors acknowledge useful discussions with Debbie Crawford, Dennis Derickson, Roger Helkey, Alan Mar, and Mark Rodwell.

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Morton eta!. 113

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