Pdf fast phenomena in semiconductor lasers researchgate. If a high pump power is suddenly applied to a laser, laser emission sets in only with a certain delay, as it starts with weak fluorescence light, which first needs to be amplified in a number of resonator round trips. Gainswitching conditions are determinant for the frequency comb quality. Increasing attention has been devoted to pulse generation from nitridebased semiconductor blue lasers by gainswitching technique 14,15,16,17, which is a simple and inexpensive technique of shortpulse generation through direct modulation of semiconductor lasers. Department of physics, university of surrey, guildford, surrey, gu2 5xh, u. These wavelengths are usually fixed once the device has been fabricated. The degree of temporal synchronization of the pulse. However, it was very difficult to obtain short pulses with duration shorter than. The gainswitched laser pulses with controllable repetitionrate from 1hz to 4khz and pulse width of 16 ns are obtained.
It is the equivalent of population inversion in a semiconductor. Effect of nonlinear gain on the bandwidth of semiconductor. Gain switching of a laser diode is easily achieved by capitalizing the first period of the relaxation oscillation which is generated by switching on a diode laser biased just below threshold56 with injected electrical or optical excitations. Pdf steppulse modulation of gainswitched semiconductor. Ultrafast pulse generation by semiconductor lasers. Gain switching occurs as a large number of carriers are initially pumped into the active region of the laser. Semiconductor lasers are essentially diodes which produces coherent light through the process of stimulated emission. Comb coherence relies on nonfull extinction of stimulated emission between pulses.
The spectral and temporal characteristics of the pulses obtained with the proposed technique are investigated with. Pdf frequency dynamics of gainswitched injectionlocked. To analyze the gainswitching dynamics of the ld endpumped nd. Gain switching is one of the most interesting methods used. The limiting factors in short pulse generation by gain switching of semiconductor lasers. Download semiconductor lasers and herterojunction leds quantum electronics principles and applications ebook free in pdf and epub format. The goal of this paper is to present and compare measured results from bulk and quantum well laser diodes utilizing the. The frequency dynamics of gainswitched singlemode semiconductor lasers subject to optical injection is investigated. From cost point of view the semiconductor lasers are economical. Emission, absorption and modal gain the analysis of a semiconductor laser diode requires a detailed knowledge of the modal gain, which quantifies the amplification of light confined to the lasing mode. Optical frequency comb generation from semiconductor lasers systematically analyzed. When the gain of a semiconductor laser is switched by a short electrical pulse from a state below threshold into gain inversion, the light emitted from the laser exhibits relaxation oscillations. This source is based on gain switching diode lasers and pulse compression schemes. Pulse widths of gainswitched pulses range from several tens to several hundreds.
This in turn requires high doping and current levels. The requirements for low time jitter and reduced frequency chirp operation. The simplest way to analyze and understand laser dynamics is using rate equations. Gain switching of a doublecorewaveguide semiconductor. Dynamics of gainswitched and modelocked semiconductor lasers. Highenergy picosecond pulse generation by gain switching in. Abstract the two processes of gain switching and modelocking in a semiconductor laser are studied within a single theoretical framework. Gainswitched semiconductor lasers are good candidates for use as seed lasers 11,12,14,15,16,17, owing to their many unique properties including their simplicity, frequencytunable capability. The simulation results show that the carrier time constants studied influence significantly the. Suppose the modal gain is ag and the facet reflectivities are r1 and r2. Gainswitched semiconductor lasers have been used as compact picosecond optical sources in many applications, such as distance measurements 1,2,3, nonlinear microscope imaging 4,5, and optical communications 6,7.
Some gas lasers can be operated with a pulsed excitation current. Once the laser is pumped above threshold, a surge of stimulated emission depletes the carriers faster than they can be injected, resulting in a very short, high power spike the gain switch spike of the pulsed dbr laser. Because laser operation starts with some low level of fluorescence light, which first needs to be amplified in a number of resonator roundtrips, the pulses are emitted with some temporal delay. The present work presents the study of an ultrashort pulse high quality laser source. Suppression of the power overshoot, typical during gainswitched operation, can be. Our conclusion is that we will have net optical gain, i. However, there are also optically pumped semiconductor lasers, where carriers are generated by absorbed pump light, and quantum cascade lasers, where intraband transitions are utilized. Gainswitched pulse generation with semiconductor lasers. One method to create ofcs is by gainswitching a semiconductor laser 410. A systematic analysis of the influence of the capture, interlevel relaxation and exciton dephasing time constants on the dynamic behavior of quantum dot fabryperot semiconductor lasers is done taking into account the lasing from the ground and excited states. The frequency dynamics of gain switched singlemode semiconductor lasers subject to optical injection is investigated. Optical injection may improve or deteriorates the optical frequency comb quality.
The gainswitching dynamics of singlemode pulses were studied in blue ingan multiplequantumwell verticalcavity surfaceemitting lasers vcsels through impulsive optical pumping. Also, semiconductor lasers can be used for the generation of ultrashort pulses with mode locking or gain switching. In this chapter, we will setup laser rate equations using the fabryperot optical cavity as a model. Highenergy subnanosecond optical pulse generation with a. Laser dynamics singlemode the larger this product the larger is the small signal gain g 0 achievable with a certain laser material. The standard gain switching arrangement is to drive a laser, biased below threshold, with an electrical rf generator. All the semiconductor lasers were pumped with impulse optical pumping, and all the generated gainswitched output pulses were well described by exponential functions in their rise parts, wherein the transient gains were. Pdf the effect of strong externalinjection into a commercial semiconductor diode to increase the laser bandwidth such that gainswitched pulses can. Gain switching is a technique for generating short optical pulses in a laser by. The model is based on the rate equations of fourlevel systems and some practical physics conditions. This is often done with tea carbon dioxide lasers co 2 lasers, which are suitable only for pulsed operation. A study of the gainswitching process in gainn mqw laser diodes is reported. Semiconductor laser is well suited for interface with fiber optic cables used in communication. Broadband tunable integrated cmos pulser with 80ps.
Read semiconductor lasers and herterojunction leds quantum electronics principles and applications. The mode of operation is somewhat di erent than that of the laser described in the previous chapter. Osa gainswitching dynamics in optically pumped single. Ultrashort pulses were observed early in the development of diode lasers. It is shown that the pulse profiles generated for each process are governed by essentially the same equations, and that the difference between the two cases lies merely in the value of a timescale. Steppulse modulation of gainswitched semiconductor pulsed laser article pdf available in applied sciences 93. Spectral dynamics of picosecond gainswitched pulses from. The gain switching of the laser using a high power sine wave signal that drives the laser gain above and below the lasing threshold is a. Single peak gainswitched optical pulses with pulse widths less than or equal to 40 ps and optical powers equal to 100 mw are observed when electrical pulses with duration of 800 ps are applied.
If one follows a guided mode through one complete roundtrip of the cavity, one finds that the change in optical power after one. Gainguided laser diodes dont mode hop, they mode ooze as the modes and gain peak shift. Gain switching is a method for pulse generation by quickly modulating the laser gain via the pump power. The temperature dependence of the gain in semiconductor lasers paul togher september 1996 a thesis submitted to the university of surrey for the degree of doctor of philosophy. Frequency dynamics of gainswitched injectionlocked. Smaller size and appearance make them good choice for many applications. Among the three main strategies for generating ofcs from semiconductor lasers, namely mode locking, gain switching and electrooptic modulation, gain switching has attracted specific attention due to its easy implementation, high flexibility in the selection of the repetition frequency and low losses.
Femtosecond pulse generation beyond photon lifetime limit in gain. The spectral and temporal characteristics of the pulses obtained with the proposed technique are investigated with numerical simulations. The temperature dependence of the gain in semiconductor. The compact, inexpensive, and easytooperate gainswitched semiconductor lasers have wide potential applications as pulse light sources in industries and medical treatments 1. The influence of the photon lifetime in the laser cavity has been examined for both types of drive, and it is shown. In lasers, mirrors form a resonator or cavity that is used to circulate light. The effect of nonlinear gain on the frequency response of semiconductor lasers at high bias powers is analysed using the multimode rate equations. The advantage of using the gain switched pulses is the. In contrast to q switching, where the resonator losses are modulated, gain switching is the generation of short optical pulses by modulating the pump power. Femtosecond pulse generation beyond photon lifetime limit. We analyzed the transient gain properties of three gainswitched semiconductor lasers with different materials and cavity structures during pulse lasing. The limiting factors in short pulse generation by gain switching of semiconductor lasers are analyzed using an approach parallel to conventional q switching analysis.
Lasers emit coherent light at wavelengths that are well defined. Gain modulation by graphene plasmons in aperiodic lattice. In many cases, distributed feedback dfb lasers are used to meet the requirements of high pulse peak power andor singlemode. The generation of picosecond optical pulses using the gainswitching method is examined using two different types of current drive at 1 ghz. The value of the material gain that satisfies the lasing condition, 2 1 1 2 r r e ag l. Rp photonics encyclopedia semiconductor lasers, laser diodes. Light is emitted at a junction between two regions of opposite doping as shown in gure 4. Figure 1b shows the shortest pulses of the gainswitched semiconductor lasers achieved in the past 40 years both via electrical and optical pumping 24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39. The initial gain and the cavity photon lifetime, respectively, limit the rise and. The gain medium is pumped by an external source, which is electrical in nature in case of semiconductor lasers 3. The temporal and spectral characteristics of the resulting pulses are compared. The average output powers in short pulses are usually limited to at most a few milliwatts, except for optically pumped surfaceemitting externalcavity semiconductor lasers vecsels, which can generate multiwatt average output. Rp photonics encyclopedia gain switching, gainswitched. Gainswitching is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, of the order of picoseconds 10.
Experimental study of optical frequency comb generation in. Semiconductor lasers types, applications, construction. Semiconductor lasers and light emitting diodes 222 variety of forms including optical, electrical, or chemical. However, the result is not necessarily gain switching in the sense of using the first emitted spike. Linear and nonlinear pulse compression for inp monolithic. Chapter 7 lasers after having derived the quantum mechanically correct suszeptibility for an inverted atomic system that can provide gain, we can use the twolevel model to study the laser and its dynamics. As a result, we can acquire the output pulse with a higher peak power the. Yvo 4 microchip laser, a theoretical model has been developed. Transient gain analysis of gainswitched semiconductor. We call the device that delivers energy to the gain medium e. To find the modal gain, one starts from the requirement that the emission as well as absorption of photons, must conserve. In spite of the strong damping of the relaxation oscillations due to nonlinear gain, bandwidths of over 40 ghz appear to be attainable in semiconductor lasers. Fp semiconductor laser is much smaller than that in a gainswitched singlemode distributed feedback. Index termssemiconductor lasers, quantum well lasers, opti cal pulses, gain switching, laser radar.
Pdf high frequency pulse generation using a gainswitched. Timing and amplitude jitter in a gainswitched multimode. Gainswitched semiconductor lasers provide an alternative solution, which have performed shortest pulses of picoseconds duration since 1970s. Theory of ultrashort pulse generation and amplification by gain. Steppulse modulation of gainswitched semiconductor pulsed laser.
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