Fourier transform spectroscopy* (FTS) is a powerful tech- nique which is widely used in the analysis of broad, complex or extremely weak spectra, especially in ...
SINGLE-MODE FIBRE FOURIER TRANSFORM SPECTROMETER Indexing terms: Measurement, Optics, Spectrometry, Remote sensing
A single-mode all-fibre Fourier transform spectrometer is described. A resolution of ~ l c m " ' (~0-7 A) is demonstrated at 820 nm. The device can be configured as a remote gas sensor.
Fourier transform spectroscopy* (FTS) is a powerful technique which is widely used in the analysis of broad, complex or extremely weak spectra, especially in the infra-red region. As a result, the technique has been extensively utilised in the fields of molecular IR spectroscopy2 and astrophysics.3 In FTS, spectral information about the source is obtained by computing the Fourier transform of the interferogram generated by scanning a mirror of a Michelson interferometer which is illuminated with radiation from the source. This letter describes details of a novel optical fibre Fourier transform spectrometer. The device can be used to provide spectral analysis of IR sources commonly used in fibre sensor and communications applications, such as diode lasers, LEDs and superluminescent diodes. Alternatively, the spectrometer may be configured as a remote gas sensor, in which the presence of certain atomic or molecular species may be determined by monitoring changes in the spectrum of the light from a broadband source caused by wavelength-selective (structured) absorption experienced on transmission through the gas sample. The basic optical configuration of the spectrometer is shown in Fig. 1. Light from the IR source (in this case a laser diode or SLD) is coupled into a fibre Michelson with 15 m long arms, one of which is coiled onto a piezoelectric cylinder (P/Z) in order to form a fibre stretcher. Light in this arm of the interferometer is reflected back towards the coupler by a fixed mirror (Ml), and butt-coupled to the end face of the fibre, whereas the light in the second arm is collimated at the fibre output and directed onto a second mirror (M2), which is mounted on a linear translation stage driven by a motorised micrometer. Light reflected by this mirror is relaunched back into the fibre and mixes with that returned in the other arm of the fibre coupler. The motorised stage is positioned such that the total optical path lengths in the arms of the interferometer can be equalised (zero optical path difference OPD) by scanning M2. The uniformity with which the path imbalance changes as the mirror M2 moves is a critical factor in determining the resolution and wavelength accuracy of the FT technique. In order to ensure that the OPD increases linearly in time when the mirror M2 is scanned, the voltage applied to the piezoelectric fibre stretcher is varied to compensate for slight fluctuations in the velocity of M2. This is achieved by directly monitoring the rate of change of the OPD, using a stable HeNe laser as a reference source; the Michelson interferometer was constructed using a 3 x 3fibrecoupler to facilitate this, and optical filtering of the Michelson output to separate the interference at 6328 A from that generated by the IR source, as shown in Fig. 1. The frequency at which fringes appear in the HeNe light at the Michelson output as M2 is scanned is directly proportional to the rate of change of the 15m fibre coil SLD
6328A filter FFT spectrum analyser reference oscillator
15m fibre coil ^f ^ cylinder
motorised translation stage
on
i'1000 V amplifier IT757T1
Fig. 1 Schematic diagram of experimental configuration
ELECTRONICS LETTERS 23rd May 1985
Vol. 21
No. 11
OPD in the interferometer. If the mirror moves at a velocity Vm, the HeNe fringes are produced at a rate 2VJ/.r, where / r = 6328 A, thus producing a photodetector signal at a frequency/0 = 2VJ/.r. Comparing this signal to a reference oscillator set at a frequency fr = 2Vj/.r, where Vm is the mean mirror velocity over its scanning range, allows slight deviations in the instantaneous value of Vm from Vm to be determined. Compensation for such fluctuations in Vm was achieved by comparing / 0 and/ r using a phase-sensitive detector (lockin amplifier), and applying voltage proportional to the phase difference to the P/Z. It can be shown that this configuration forms a basic phase-locked loop, which is capable of tracking a step change in Vm with zero frequency error (i.e. with/ 0 =fr constantly maintained). This feedback loop ensures that the OPD of the interferometer increases linearly with time when the mirror M2 is scanned. The interference generated at the Michelson output by the IR source is monitored by a photodiode, and fed directly to a fast Fourier transform spectrum analyser (HP 3582A). The frequency spectrum recorded by this analyser represents the optical spectrum of the source, with a scale factor between wavenumber k and measured frequency/given by k =/[/c r // r ], where kr = 15 802-3 cm"1 is the wavenumber of the HeNe light. The fundamental resolution of the spectrometer in terms of wavenumber is given by (l/2d) cm" 1 , where d is the maximum displacement of the mirror M2 from the zero OPD position of the interferometer (in centimetres). The performance of the spectrometer is demonstrated by the spectra recordings shown in Fig. 2, which were obtained from a fibre-
Fig. 2 Spectrometer output of a single-mode GaAlAs laser at various injection currents
Threshold = 34 mA Note that/, = 420-25 Hz {kjf, = 37-60 cm" 1 Hz"1)
pigtailed laser diode source (M/A Com LTD 363), operated at various injection currents. The scanning mirror displacement used here was ~6-5 mm, giving a resolution of ~0-7 cm"1, which after electronic apodisation was reduced to ~1 cm" 1 (corresponding to ~0-7 A at the laser operating wavelength of ~ 828 nm). The use of a scanning mirror in the spectrometer is a serious disadvantage, as the light has to be collimated and relaunched back into the fibre after reflection of the moving mirror. It is, however, possible to introduce the desired path imbalance using a completely all-fibre system, by directly utilising the P/Z fibre stretcher or, alternatively, by heating one of the fibre arms. Our P/Z fibre stretcher was capable of producing an effective OPD change of ±1-5 mm maximum, with Vapplied = ± 1000 V, which set the resolution at 463
~ 4 cm \ However, the use of more fibre, and P/Z stretchers in both arms, would allow this resolution to be substantially improved. Heating an optical fibre may also be used to produce a relatively large change in the effective optical path length in the fibre. This arises primarily as a result of the temperature dependence of the fibre refractive index, although thermal expansion also contributes.4 Optical path length changes of the order of 10/rni/°Cm (100 rad/°Cm at X = 0-63 /im) are typical of the magnitude of this effect. The feasibility of the temperature-scanning technique was tested using the optical set-up previously described. The P/Z fibre stretcher in one arm was retained, and the fibre in the other arm was wound onto a nylon former and placed under a thermal source. The mirror M2 was fixed in the position corresponding to zero OPD. The fibre heating rate was adjusted such that the mean frequency at which the HeNe fringes were produced during the heating cycle matched the reference oscillator frequency/,. The P/Z fibre stretcher feedback loop was then connected to compensate for the nonlinear heating of the fibre. The compensation range of the P/Z fibre stretcher was found to be sufficient to correct for the nonlinear heating rate over a period of up to 50 s. Results obtained using an SLD source with the spectrometer operated in the temperaturescanning and mirror-scanning modes showed a high degree of correlation, although a certain level of apodisation was caused by birefringence effects in the temperature-scanning mode, which resulted in a slightly reduced resolution. The possibility of configuring the fibre spectrometer as a remote gas absorption sensor is shown schematically in Fig. 3.
3 CONNES, J., and CONNES, P.: infrared astronomy' (Gordon & Breach, New York, 1968) 4 HOCKER, G. B.: 'Fiber-optic sensing of pressure and temperature', Appl. Opt., 1979, 18, p. 1445
OPTICAL FIBRE FARADAY ROTATION CURRENT SENSOR WITH CLOSED-LOOP OPERATION Indexing terms: Measurement, Opticalfibres,Optical sensors A novel closed-loop fibre-optic current sensor based on the Faraday effect is described which maintains a constant sensitivity to AC currents in the presence of large quasi-DC and low-frequency current fluctuations. A Verdet constant of 2-55 x 10~6 rad/A (at 0-83 nm) is reported for thefibreused.
The magneto-optical Faraday effect in optical fibres has been utilised as the basis of several fibre-optic sensors and devices, such as magnetometers,1 current sensors2"4 and optical isolators.5 Current sensing using the Faraday effect in fibres is a particularly attractive technique, especially for applications involving high-voltage power line monitoring. The technique may also prove valuable for sensing applications in environments subject to high levels of electromagnetic noise. The polarisation rotation
