Efficient Frequency Noise Reduction of GaAIAs Laser Diodes by Negative Electronic Feedback

Abstract

Negative electronic feedback (NEF) is a promising technique to improve the coherence properties of semiconductor laser diodes (LDs). The system considered here consists of a LD (HLP 1400), a frequency discriminator (ring cavity, round-trip length 13 cm, linewidth ≈ 80 MHz FWHM) and a servo amplifier. If properly designed, NEF reduces the frequency noise spectral density S of the free running laser (index FR) at Fourier frequencies f below a certain value fu (≈ servo loop unity gain frequency), following more or less a 20 dB/decade-dependence. We will assume that the 1/f-corner frequency of SFR is smaller than fu, i.e. SFR is almost constant in this frequency region. Below a lower corner frequency fl, SNEF, becomes almost frequency independent in many practical cases. This noise limited level SNEF(f<fl) and the total phase noise power PNP(fl), defined as the integral over all phase noise components at Fourier fre-quencies above fl, are the most important characteristics of a NEF set-up and influence sensitively the spectral properties of the LD. PNP(f1) has to be less than 1 rad2 in order to keep the total optical sideband power smaller than the central line (“carrier”) power. This condition can be fulfilled only with sufficiently short feedback loop delay times. Our computer simulations show that this time should be less than 6 nsec for a HLP 1400. We used 3.2 nsec during the experiments. SNEF(f<fl), called SMIN in the following, characterizes the width ∆υ of the lorentzian central line: ∆υ = π • SMIN• Here we have assumed SMIN<<fl • SMIN may depend on different noise contributions: 1. Shot-noise (SN) is of minor interest if linewidths of more than 1 kHz are considered. For example the SN contribution to SMIN of a typical set-up (10 μW at the photodetector, discriminator slope of 1/70 MHz, as in our case) is of the order of 100 Hz2/Hz, as indicated in Fig. 1. 2. Electronic noise contributions to SMIN can be kept below the SN-level by using sufficiently large photocurrents (>10 μA), small area photodiodes and low noise transimpedance preamplifiers. 3. Relative intensity noise of the dominant longitudinal LD-mode, named RIN(DM) in the following, may be a severe problem, at least for LDs of Fabry-Perot type. It is well known, that RIN(DM) is usually several orders of magnitude higher than the all-mode noise RIN(TOT) for these lasers. For example, we found for our CSP GaA1As LD operating in a nearly single longitudinal mode: RIN(DM) = 5 • 10−10 Hz−1, RIN(TOT) 100 kHz. It is obvious, that a conventional two beam slope discriminator, where the unfiltered LD output signal is used as the reference for AM noise suppression by electronic substraction from the Fabry-Perot transmission signal, is inadequate for this application because a substantial part of RIN(DM) is converted to FM noise by the control loop. The resulting equivalent SMIN-level of about 3 • 105 Hz2/Hz is indicated by RIN(DM) in Fig. 1. We reduced this noise contribution by at least three orders of magnitude by using a resonator with internal birefringent plate. Both sets of orthogonally polarized, frequency shifted, resonator eigen-modes were excited equally by choosing a proper input polarisation. The frequency shift was adjusted to about one full cavity linewidth by applying a transversal force to the intracavity plate (stress birefringence). Then, the photocurrent difference of both orthogonally polarized transmission signals displayed dispersion-like spectral features of odd symmetry, which are needed for servo control. Both transmission signals, which were detected by separate photodiodes, carried almost the same amplitude modulation RIN(DM) while AM contribution from the longitudinal neighbour modes was highly suppressed. This scheme results in an efficient RIN(DM) compensation for the difference signal. We measured a RIN(DM) suppression of better than −30 dB, as indicated in Fig. 1, leading to an equivalent SMIN value of about 300 Hz2/Hz. 4. AM/FM cross modulation noise (CM-noise) is defined here as the mixing product of AM and FM noise components having a frequency difference of less than fl. We estimated the equivalent value of SMIN due to this contribution to be of the order of 104 Hz2/Hz under our conditions, see CM-noise level in Fig. 1.