Mode locked lasers are a compact, inexpensive source of optical pulses. Currently, some problems with semiconductor sources are low output power, self phase modulation, secondary pulse formation, and timing jitter. These problems are addressed here

Absorption recovery dynamics of GaAs/AlGaAs field-enhanced waveguide saturable absorbers are studied by pump-probe differential transmission measurements. We compare the response of bulk and single quantum well absorbers at different reverse bias levels and pump powers, and find an ultrafast transient in the response, followed by a slower rise before the final recovery. The absorption fully recovers after a few picoseconds, which is an important result for mode-locked lasers.

Semiconductors have recently been shown to be useful as saturable absorbers. Both bulk and quantum well materials have been used to generate picosecond and femtosecond pulses in color center[1], erbium-doped fiber[2], dye[3] and semiconductor[4] laser systems. These applications typically use damage sites or traps to decrease the absorption recovery time. In semiconductor diode lasers, monolithic structures incorporating waveguide saturable absorber sections for passive modelocking have produced pulses of 0.6 picoseconds, at repetition rates up to 350 GHz. However, the exact mechanism for such high frequency modelocking is not well understood. In order to produce a stable train of short pulses at these millimeter-wave repetition rates, the absorption recovery must be fast enough to recover on each round trip in the cavity, e.g., 10 ps for a 100 GHz repetition rate. The saturable absorber sections are typically p-i-n structures operated under reverse bias to remove photogenerated carriers and achieve this fast recovery, as in a p-i-n photodiode. Knowledge of the dynamics of the saturable absorbers is necessary for design optimization, as well as insight into the potential of these devices for producing short optical and electrical pulses. Absorption recovery is also thought to be responsible for elimination of multiple pulses in external cavity lasers[5]. In this paper we will present a study of the bleaching and recovery dynamics of these waveguide saturable absorbers for different bias conditions, and discuss the implications of the results for modelocked laser systems.

Methods of millimeter-wave signal generation are presented using frequency multiplication by optical comb generation, direct generation using optical oscillators, and mixing between CW optical sources.

Mode-locked vertical cavity lasers have a large cross-sectional area and consequently a large saturation energy and large peak powers. The authors analyze excess optical bandwidth generation in these lasers and find that self-phase modulation due to optical pumping and gain saturation is the dominant factor in inducing laser pulse chirping. The large magnitude of the chirp makes intracavity prism-pair compensation difficult. Adjustment of the cavity length has a major impact on the pulse chirping, as observed experimentally. Proper adjustment can result in a large linear frequency chirp which can be compensated using external pulse compression techniques. >

Monolithic and external cavity mode-locked semiconductor lasers with multiple contacts have produced excellent results [1–3]. The goal of this paper is to show how to optimize the design of these structures as a function of the free parameters in the design: (1) Active region composition and waveguiding, (2) segment lengths, and (3) electrical parasitics. The paper also explains how the effects of self-phase modulation in the saturable absorber and gain section interact with gain dispersion to limit the achievable pulse width. Figure 1 shows the types of structures that were fabricated and analyzed in this work. Both quantum well and bulk active region devices were tested at 0.85 μm and 1.55 μm. The functions of the various segments are: segment A provides the overall gain, segment B is reverse biased to form an intra-waveguide saturable absorber/photodetector, and segment C is used for active gain modulation or for repetition rate tuning.

Mode-locked semiconductor lasers which incorporate multiple contacting segments are found to give improved performance over single-segment designs. The functions of gain, saturable absorption, gain modulation, repetition rate tuning, wavelength tuning, and electrical pulse generation can be integrated on a single semiconductor chip. The optimization of the performance of multisegment mode-locked lasers in terms of material parameters, waveguiding parameters, electrical parasitics, and segment length is discussed experimentally and theoretically.

Research on integrated optical devices has often focussed on complex structures and complicated processing techniques [1,2]. However, a variety of integmted optoelectrOnic devices can be fabricated fiom a simple semiconductor laser diode waveguide process. Several important functional components can be obtained by splitting the top contact in a laser process into several segments allowing for non-uniform pumping. When forward biased, the device has gain allowing far amplification, direct current modulation of the gain, and modulation of the index of refraction. When reverse biased, the segment functions as a p-i-n photodetector, saturable absorber, and electro-absorption modulator. With these simple functions, many useful integrated optoelectronic devices can be formed. Figures la and lb show example transmitter and receiver devices which have been fabricated with our process. Figure la shows a three-segment monolithic cavity mode-locked semiconductor laser 131 with a waveguide saturable absorber, gain modulation segment, and gain segment. Devices with repetition rates as low as 5.5 GHz (7mm device length) and as high as 80 GHz (0.5 mm device length) have been fabricated. Figure 2 shows an autocorrelation trace for a 1.55 pm wavelength monolithic cavity mode-locked laser operating at a repetition rate of 21 GHz. Device structures similar to that of Figure la have been used for a 3-section gain-switched laser in which two saturable absorbers near the laser facets are used to obtain shorter pulsewidths than are possible with single-section gain-switched lasers. The device of Figure la can also be used as a two-segment superluminescent LED. A short reverse-biased segment acts as an optical termination to absorb spontaneous emission in one direction with a power reflection coefficient below lo4. The reverse-biased optical termination segment also serves to monitor average power from the superluminescent LED. Figure lb shows a 3 segment pre-amplified photodetector in which one segment is used as an optical amplifier, the second segment is used as a switch to turn the signal to the detector on or off, and the third segment is the p-i-n photodetector. Figure 3 shows the impulse response of the waveguide photodetector with a mode-locked semiconductor laser as the pulse source. The full width at half of maximum is 33 ps limited by the capacitance of the photodetector. The switching segment can be tumed on and off in 200 ps using a step-recovery diode drive signal. The switching segment can also be used as a saturable absorber in pulse amplification applications. The entire spontaneous emission output from the amplifier does not reach the photodetector between optical pulses. Pumpprobe measurements of the saturable absorber section were made to measure the absorption recovery time constant. The saturable $sorber can recover to the highly attenuating state in less than 10 ps after the passage of the optical pulse through the absorber denonsytting that the saturable absorber can be effective in reducing the spontaneous emission reaching the photodetector. An outline of a low capacitance multi-segment laser process is shown in Figure 4. A silicon nitride layer is deposited and patterned to outline the waveguide. The silicon nitride is used as an etch mask to define the waveguide for wet or dry chemical etching. Next, a polyimide layer is deposited and cured. In order to planarize the polyimide layer, a thick layer of photoresist is spun over the entire wafer. Both the polyimide and the photoresist layers are then etched in an oxygen plasma until the top of the m m slightly protrudes through the polyimide layer. The silicon nitride layer is then removed and the wafer is ready for metallization. The multi-segment p-metal is evaporated, lifted off and annealed. The metal contacts are used as a 3-way self-aligned mask to 1) etch the conmt layer between electrodes, 2) etch cleave marks for precise positioning of facet cleaves, and 3) remove the excess polyimide in order to promote good facet cleaving. Finally, the wafer is thinned, a backside metal is deposited and the devices are cleaved. [ 11 U. Koren, Indium Phosphide and Related Materials Conference, April, 1990. Paper TuC.1. [2] T.L. Koch, U. Koren, R.P. Gnall, F.S. Choa, F. Hernandez-Gil, C.A. B m , M.G. Young, M. Oron, and B.1 Miller, Indium Phosphide and Related Materials Conference, April, 1990. Paper TuC.2. [3] D. J. Derickson et al., 1992 Optical Fiber Conference, Paper ThB3. San Jose, Ca

Imperfect antireflection coatings in external-cavity mode-locked semiconductor lasers can cause multiple output pulse generation. The incorporation of an intrawaveguide saturable absorber segment into the laser suppresses this problem. Single pulse outputs of less than 2.8 ps and 0.7 pJ of energy are obtained using such devices with both quantum well and bulk active regions.<>

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text D. J. DERICKSON, R. J. HELKEY, A. MAR, J. G. WASSERBAUER, W. B. JIANG, and J. E. BOWERS, "Modelocked Semiconductor Lasers: Short Pulse, Small Package!," Optics & Photonics News 3(5), 14-20 (1992) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

The repetition frequency of an external cavity mode-locked GaAs semiconductor diode laser has been stabilised by voltage controlled electrical feedback. The phase noise has been reduced by 40 dB at 1 kHz offset from the carrier and timing jitter reduced from more than 30 ps to 4 ps. This technique can be used to stabilise millimetre-wave mode-locked lasers.<>

The authors consider electrical and optical signal generation techniques using passively and hybridly mode-locked semiconductor lasers with intra-waveguide saturable absorbers. Microwave and millimeter-wave signals can be generated using mode-locked semiconductor lasers with the intra-waveguide saturable absorbers. Monolithic and external cavity devices optimized for electrical and optical signal generation are discussed along with measurements of amplitude noise, phase noise, output power, and repetition rate tunability.<>

A comprehensive timing jitter comparison is made for mode-locked semiconductor lasers using active, passive, and hybrid mode-locking techniques in both external and monolithic cavity configurations. Active mode locking gives the lowest residual rms timing jitter of 65 fs (150 Hz-50 MHz), followed by the hybrid and passive mode-locking techniques. It is found that monolithic cavity devices with all active waveguides have higher timing jitter levels than the comparable external cavity case.

Mode-locked semiconductor lasers have produced ultra-short pulses in external cavity configurations by both active and passive methods. Recently, progress has been made1–3 in combining all of the required components into a single monolithic mode-locked device. These devices operated at repetition rates above 15 GHz at 1.3 μm and 1.55 μm wavelengths. Formany applications such as electrooptic sampling, A/D converters, and OTDRs, lower repetition rates are usually necessary. The devices studied in this work have a repetition rate of 5.5 GHz at a lasting wavelength of 0.85 μm.

The magnitude and origin of the pulse-to-pulse timing jitter that is added to the modulating source by an actively modelocked laser is discussed. Residual and absolute RMS timing jitter measurements as low as 50 fs and 170fs, respectively, have been measured in optimally tuned cavities.

A new mode-locking technique, self-mode-locking, is described which uses the detected optical pulses from the mode-locked laser as the active driving source. This technique forms narrow-width mode-locked optical pulses at low repetition rates without the use of a microwave synthesizer.