Tutorial Review, Optical and Quantum Electronics, February 1995, Volume 27, Issue 2, pp 63-89
ABSTRACT
The classical theory of laser noise treats light in a classical manner, yet agrees with quantum theory for large particle numbers. The basic concept is that laser noise is caused by atomic jumps between lower and upper levels, and that atoms subjected to classically-prescribed optical fields are independent. The treatment of amplitude noise of single-mode cavities containing resonant three-level atoms is applicable to semiconductor lasers at moderate power. At high power one must account for the dependence of the gain on optical power and for state-occupancy fluctuations. The phasor theory that attributes noise to the beat between the oscillating field and the field spontaneously emitted in the mode by excited-state atoms cannot be understood consistently in semiclassical terms.
Appl Opt. 1994 Oct 20;33(30):6947-52. doi: 10.1364/AO.33.006947.
ABSTRACT
We present an electrical model for modulation and noise of laser diodes that takes spectral-hole burning into account and, unlike previous models, is accurate at the quantum level. The active part of the laser diode is represented by a capacitance-expressing carrier storage and a series resistance 1 + β, where β is proportional to the spectral-hole depth. These two elements are followed by a negative impedance converter. The modulation rate measured on this electrical model is in excellent agreement with the theoretical expression. Amplitude noise is simulated by two independent noise sources whose spectral densities are independent of the nonlinearity.
A simple expression for the linewidth of vertical cavity, periodic gain, laser diodes is reported. Quantum wells, spaced half a wavelength apart are assumed to be driven by time-independent electrical currents J k, and to possess distinct phase amplitude coupling factors αk. The usual linewidth formula is found to be applicable if the photon lifetime is defined as the round trip time divided by a factor of 6, and the averaging of the αk is made with Jk as a weighting factor. Furthermore, half the spatial variance of α must be subtracted. The expression involves cross terms of the form αiαj, i≠j
The theory presented shows that light emitted by low-temperature semiconductors under intense optical pumping (with fluctuations at the shot-noise level : SNL) should be amplitude-squeezed down to half the SNL at nonzero frequencies. Amplitude squeezing may be obtained also at zero frequency when spontaneous carrier recombination is significant. It is essential that the optical gain depend on photon emission rate, E. G., as a result of spectral-hole burning. A commuting-number theory that agrees exactly with Quantum Theory for large particle numbers is employed. Comparison with results previously reported for 3-level atom lasers is made.
Physical Review A, volume 48, number 3, september 1993
ABSTRACT
Spectral-hole burning (SHB) profoundly affects the modulation and noise properties of laser oscillators and amplifiers at high optical power. The present semiclassical theory of SHB shows that the optical gain should be considered a function of the carrier and photon rate (rather than photon number) plus a fluctuation at the shot-noise level (for full population inversion). Constant-voltage-driven laser diodes generate amplitude-squeezed light, a result not predicted by previous theories that treat gain compression in a formal way. Amplitude and phase noise of oscillators and amplifiers are considered.
Electronics Letters, Volume 29, Issue 15, 22 July 1993, p. 1320 – 1322
ABSTRACT
A new kind of phase-insensitive optical amplifier is proposed whose output is in the coherent state (ideal laser light) if the input is in the coherent state. In-phase and quadrature modulations are preserved in absolute values. Both modulations can be tapped off. The amplifier employs conventional optical amplifiers, electrical feedback, and all-pass filters. Remarkably, these properties hold when either linear or nonlinear optical amplifiers are employed.
Absorbing or emitting elements generate noise waves. The main purpose of this paper is to determine from first principles the spectral density of noise waves relating to nonlinear elements. This was done by considering the combination of linear elemnts (whose noise properties are well understood) ans lossless circuits that are nonlinear because of the Kerr effect. Lossless nonlinear circuits transform noise waves but do not generate noise. A semiclassical theory shows that noise waves remain at the shot noise level (for full population inversion) if the optical gain is considered a function of photon rate (rather than optical intensity). The result, in extact agreement with an independent theory of spectral-hole burning, is conjectured to be general. Intensity fluctuations of a Kerr oscillator are squeezed below the shot-noise level for large Kerr constants.
Physical Review A, vol 45, number 3, February 1992
ABSTRACT
The small-signal modulation and noise properties (electrical voltage, optical power and phase) of laser diodes depend on ten real parameters relating to the semiconductor material employed. Among these, the phase-amplitude coupling factor α is of particular importance. These parameters are evaluated for GaAs at 0.87 μm, GaInAsP at 1.55 μm and InAsSb at 3.87 μm at room temperature. Revised expressions for the optical gain are used. The light-hole contribution, the plasma effect and band-gap shrinkage are taken into account. The latter leads to a significant reduction of α, particularly below the peak-gain frequency. The α-factors for the three materials listed above are found to be, respectively, 2.9, 3.85 and 8.3 for conventional diodes
Electronics Letters, Volume 27, Issue 25, 5 December 1991, p. 2354 – 2356
ABSTRACT
An exact yet simple expression for the linewidth of laser diodes based on the Nyquist formula is given. The expression applies to the case where the optical gain depends on both the carrier and photon numbers and differs from the expression derived from standard rate equations. Gain compression in conjunction with an electrical conductance may reduce both the linewidth and the intensity noise.