AFTERNOON. THE4 Fiber-optic measurement instrument for orders of magnitude lower in amplitude.' However, temperature when these devices are coupled to ...
THE4 Fiber-optic measurement instrument for temperature
ordersofmagnitudelower in amplitude.'However, when these devices are coupled to fiber interferometer systems, deterioration of their emission K. KYUMA, S. TAI, T. MATSUI, T. SAWADA, and M. characteristics can limit their application as sources NUNOSHITA, Mitsubishi Electric Corp., Central for sensitive interferometer sensors. Deterioration occurs when a small amount of external feedback Research Laboratory, 80 Nakano, Minamishimizu, is coupled into the laser cavity and from the lack of Amagasaki, Hyogo, 61 1 Japan. frequency stability which appears inherent in To observe the dynamic conditions of high-power semiconductor laser devices. electric machines, fiber-optic measurement systems Frequency instability manifests itself in the form are strongly indicated. In this work, we propose a of low-frequency phase noise in interferometer unique fiber-optic thermometer constructed of a systems. This effect is observed in both gain-guided temperature-sensitive semiconductor sensorand and index-guided lasers. Phase-noise measurea signal processing system with various wavelength ments were made using an unbalanced Michelson LED sources. interferometer with a variable optical path difference The principle of measurement is based on the (OPD).The data taken with an Hitachi HLP 1400 band-edge shift of the semiconductor absorber with CSP laser are typical of several different laser temperature. Two types of sensors were constructures, and phase noise is seen to vary linearly structed. In one type, a 150-pm thick semi-insuwith OPD. This behavior is explained by low-frelating GaAs chip is sandwiched between two fiber quency fluctuations in the lasing wavelength of the ends. This system exhibits a high sensitivity for order of nm for a IO-Hz bandwidth at 1 kHz. temperature. The other system is constructed of This behavior reduced the sensitivity and dynamic a 2-mm thick polycrystalline CdTe chip and two range of aninterferometer sensor with a 40cm OPD %-pitch graded-index rod lenses. These lenses by a factor of IO3. terminate the incoming and outgoing fibers. The Small amounts of feedbackof the order of 0.01 % CdTe sensor is useful over a wide temperature resulting from coupling the laser to a fiber-optic range. The system is illustrated schematically in system can induce mode swapping between adjaFig. 1. A pair of optical sampling pulses of different cent longitudinal modes. This phenomenon, while wavelengths XI ( ~ 0 . 8 7 pm) and iz ( E 1.27 pm) is a sensitive function of phase, can increase the guided from GaAs-LED and InGaAsP-LED through the low-frequency noise several decibels above thefree sensor and optical fibers to a detector. The abrunning noise level of the laser. These data are sorption of by the sensor is a function of temseen in Fig. 1 and represent the maximum noise perature. On the other hand, these semiconductors obtainable by varying the phase ofthe light from the are transparent to i2, which we use as a reference external cavity as the amount of feedback was insignal. After conversion to electric signals by a creased. Ge-APD, the output of the temperature-dependent External feedback of >0.01 YO can reduce the signal is normalized by that of the reference spectral qualities of the laser output aswell. In CSP signal with an electric divider. The divider lasers feedback of >0.01% induces satellite modes output is dependent only upon the band-edge shift symmetrically located about the primary mode and of the semiconductor. shifted by 1-4 GHz. The spacing of these modes Figure 2 shows the observed relations between in frequency from the primary mode was indepenthe divider output voltages Vo and the temperature dent of the external cavity spacing but was depenin the -30 to +8OoC range. From the stability of dent upon the forward driving current of the laser. Vo. the precision is estimated to be within 310.4 and With increased feedback (above 0.1%) all lasers + l 0 C for the GaAs and CdTe absorbers, respecdemonstrated an increase in emission linewidth by tively. InFig.3, the deviation of divider output a factor of -40. Such feedback-induced line voltage VO,temperature-dependent signal voltage broadening reduced the coherence length from 60 Vs. and reference signal voltage V, is shown as a m for the free running laser to a few centimeters. function of the optical connector loss. This figure The variation of linewidth with feedback is shown in shows theexperimental results at room temperature Fig. 2. in the case of the CdTe absorber. The change of These properties and others limiting the use of V, is