Modulation response of dynamic single-mode quantum-well heterolasers in the 1.5 μm range

Abstract

Numerical modelling is used to study the effect of tuning the laser output over the gain bandwidth on the modulation response of GaInAs-GaInAsP quantum-well heterolasers for different modulation frequencies of the pump current. It is found that the maximum frequency bandwidth of the response band and the greatest feasibility of high speed modulation for transmission of signals in information systems are achieved in the center of the gain band. Raising the dc component of the pump current increases the response bandwidth. For typical parameters of this system (near 1.5 μm) the maximum response bandwidth can approach ≈40 GHz.For certain parameters, the amplitude-frequency characteristics of the heterolasers have two local maxima: one at low frequency corresponding usually to a resonance for the 1/2 subharmonic and one at high frequency, for the fundamental resonance.Introduction. Solving a number of scientific and practical problems requires single-mode (single frequency) semiconductor lasers with a narrow and stable emission line. Furthermore, in a number of cases the frequency of the laser radiation must be tuned over a certain range within the confines of the gain band. As a rule, single frequency operation is obtained using an external dispersive cavity. For example, narrow band cw lasing (Δλ ≤ 0.15 nm) at an output power of up to 20 mW tuneable over an interval of ≈10 nm around the central wavelength of 884 nm has been obtained [1] in a GaAs-AlGaAs heterolaser with an external dispersive cavity at room temperature. Single frequency emission from a laser with stressed active layers (λ = 975 nm) and a cw power of >100 mW has been obtained [2] with suppression of side longitudinal modes at a level of >30 dB. It has been pointed out [3] that with the right position for the fixed axis of rotation of an external mirror, an injection laser can essentially be tuned continuously over its entire gain band. An external waveguide grating mirror has been used to obtain a significant narrowing (to 0.1 nm) of the output spectrum and it was shown that the output wavelength of a semiconductor laser can be tuned smoothly over a range of 10-18 nm [4].It has been noted [5] that one of the most important problems in fiber optic communications lines with channel spectral densification is the creation of laser sources with a narrow and stable spectrum (no more than 0.1 nm) that must be maintained in a dynamic regime with modulation of an information signal at frequencies up to 10 GHz. These requirements may correspond best to semiconductor lasers with external fiber Bragg gratings on single-mode fiber light guides. For GaInAsP-InP lasers the half width of the laser output spectrum with a fiber Bragg grating is no more than 0.1 nm with pulsed modulation of the pump current at a frequency of up to ≈5 GHz. Other features of the output parameters of single frequency semiconductor lasers are discussed in [6][7][8][9][10][11]. Methods of creating dynamic single-frequency semiconductor lasers that operate stably to generate a single longit...

Selection of the lasing frequency and heating the quantum-well heterostructure laser diodes

“…These emitters based on the quantumwell (QW) GaInAs-GaInAsP heterosystem are the principal sources for fiber-optical communication networks. As shown, at typical laser diode parameters due to spectral selectivity it is possible in the range of 1.5 μm to extend and get the modulation response bandwidth up to ≈ 40 GHz [4].…”

“…The laser amplitude-frequency characteristics at definite parameters display two local maxima. Low-frequency maximum corresponds ordinary to sub-harmonic 1/2, and high-frequency one to the main resonance [4]. Mention that at a powerful excitation, where heating is essential, the modulation depth of output radiation changes versus the modulation frequency ν m .…”

“…Besides, in a number of cases it is necessary to tune the lasing radiation frequency within the gain band. In particular, an important interest is a systematic research of output parameters of semiconductor lasers with a high-frequency pump current modulation and selection of the lasing frequency, which is used in fiber-optical information systems [3,4]. One from the main problems of fiber-optical communication with wavelength-division multiplexing is development of laser emitters with line of width no more than 0.1 nm and maintaining the modulation signals with frequencies up to 10 GHz.…”

“…It is considered that lasing occurs at a single longitudinal mode with the frequency ν st , which can to displace at any point of the gain band. To describe operation of the laser, use the rate equation system for the photon density S in the cavity and for the nonequilibrium current carrier concentration N, similarly to [4]…”

“…where v is the light velocity in the active region, μ is the coefficient of fulfillment of the active medium by cavity, Γ is the confinement factor, k(ν st ) and k l are coefficients of gain and losses, R sp is the rate of spontaneous radiative recombination, β is the coefficient determining the contribution part of spontaneous radiation into lasing mode, j is the pump current density, d is the thickness of the active layer (QW width), N a is the number of QWs in the active region, η' is the injection efficiency, η sp is the quantum yield of luminescence. The gain spectrum k(ν st ) and rate of spontaneous radiative recombination R sp are determined in the framework of the optical model of the active medium with no the k-selection rule on the electron wave vector for ground transitions [3,4]. Modulation of pump current is described in the form, as j = j b + j m sin(2πν m t), where j m and ν m are depth and frequency of modulation, and j b is the direct current constant component.…”