Experimental method based on wavelength-modulation spectroscopy for the characterization of semiconductor lasers under direct modulation

Abstract

An experimental method is presented for characterization of the combined intensity and frequency modulation produced when the injection current of a laser diode is modulated. The reported technique is based on the analysis of the harmonic signals produced when a modulated laser is used to probe a gas absorption line by the so-called wavelength-modulation spectroscopy method. Based on a theoretical model of this technique, we present two methods that facilitate the determination of ͑i͒ the deviation in laser frequency and ͑ii͒ the phase shift between intensity and frequency modulation. These methods are illustrated experimentally by measurement of the modulation parameters of a 2-m distributed-feedback laser by use of a CO 2 absorption line. The experimental results have been compared with those obtained with another traditional method and have shown full agreement in the frequency range ͑400 Hz-30 kHz͒ considered.

Laser offset-frequency locking up to 20 GHz using a low-frequency electrical filter technique

“…Good qualitative agreement is observed, but quantitative discrepancies occur for the same reason as previously mentioned. Here again, the observed behavior is very similar to the situation experienced when a molecular absorption line is probed by a wavelength-modulated laser: the amplitude of the 1f signal varies periodically with the phase Φ 1 of the lock-in detection and the signal of maximum amplitude is obtained for Φ 1 ¼ Ψ as in WMS, where Ψ is the IM-WM phase shift [15]. The only but important difference is that the 1f signal is free from background offset for any detection phase, whereas such a background offset is usually present in the 1f -WMS signal as a result of the laser residual IM occurring when the current of a semiconductor laser is modulated [16].…”

Frequency modulation characteristics for interband cascade lasers emitting at 3 μm

“…10b by choosing the reference point for 0 dB to be 500 Hz. The 3 dB cutoff frequencies of the 2997 nm ICL and the 3266 nm ICL are 80 kHz and 10 kHz, respectively, which are higher than those of QCLs (i.e., ~400 Hz) [12] and 2-μm DFB DLs (i.e., ~4 kHz) [8]. Furthermore, the 6 dB cutoff frequencies of the 2997 nm ICL and the 3266 nm ICL are 300 and 100 kHz, respectively.…”

“…Note that we introduce the FM/IM phase shift ψ 1 in the IM rather than in the FM of the laser. If the phase shift ψ (called the IM/FM phase shift) is introduced in the FM, as described in the literature [5,8], the relationship between ψ and ψ 1 can be written as follows:…”

“…10a, the typical absolute value of the current rate of ICLs at the same modulation frequency is at least four times that of NIR DFB lasers, which makes ICLs attractive for direct absorption spectroscopy of species with broadened spectral features (e.g., caused by large Frequency (Hz) 2997nm ICL 3266nm ICL 2 m DFB laser by Ref. [8] (b) 3dB molecules or high gas pressures). One has to note that the detection speed may decrease to only several kHz for gas species with broadened spectral features.…”

“…Schilt et al [8] measured the FM efficiency and IM/FM phase shift of a 2.0-μm distributed feedback (DFB) diode laser (DL) using an unbalanced interferometer and the WMS method with the R16 absorption line of CO 2 . The results showed that the two parameters were inversely proportional to the modulation frequency (100 Hz-100 kHz).…”