40 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 1, JANUARY 1, 2006 Standing-Wave Fourier Transform Interferometer With an HPT Jun-Xian Fu, Member, IEEE, Xiaojun Yu, Bingyang Zhang, and James S. Harris, Jr., Fellow, IEEE Abstract—A compact standing-wave Fourier transform inter- ferometer is experimentally demonstrated in a very broad band. By using an InP–InGaAs–InAlAs heterojunction bipolar photo- transistor (HPT) and a PZT-controlled scan mirror, a resolution of 37.5 cm was achieved with a mirror travel length of only 32 m at the fifth harmonic order spectrum component. The experimental results agree well with the mathematical models, and the interferometer resolution could be further improved. Index Terms—Fourier transform, heterojunction bipolar photo- transistor (HPT), InGaAs–InP photodetector, interferometer, re- mote sensing, telecommunication. I. INTRODUCTION A STANDING-wave Fourier transform infrared interfer- ometer (SWFTIR) is a highly compact system designed to implement field applications of sensing and analysis of co- herent light sources with reasonable resolution [1]. The general system only includes two elements, a partially transparent photodetector and a scanning mirror. A phase shift between the incoming wave and reflected wave is introduced at the ideal conductive mirror surface, and a standing wave will be formed on the condition that the light sources are coherent. Compared to the original form of the Michelson spectrometer, the SWFTIR eliminates the 50/50 beam splitter and second fixed mirror arm and can be easily integrated into an array system for hyperspectral sensing and imaging. The system was first proposed by Miller [1] and later demonstrated in the same group with a bulk micromachined silicon microelectrical–me- chanical system (MEMS) mirror and a thin-film detector with a resolution as good as 6 nm for 633-nm red laser sources for a mirror scan length of 32 m [2]. Although the SWFTIR has been proposed for identification of low-coherent light sources [3], the compact system is more advantageous for identifi- cation of highly coherent light sources because the distance between the detector and the scan mirror is limited by MEMS processing technology and available operating conditions. The displacement range that the mirror can scan is dependent on the coherent length of the light sources. By scanning the highly reflected mirror or the partially transparent photodetector, the intensity of the standing wave is sampled. The photocurrent Manuscript received August 3, 2005; revised September 3, 2005. This work was supported in part by the Defense Advanced Research Project Agency under the PWASSP program. J.-X. Fu was with the Solid State and Photonics Laboratories, Stanford Uni- versity, Stanford, CA 94305 USA. He is now with Exponent Failure Analysis Associates, Inc., Menlo Park, CA 94025 USA (e-mail: jfu@exponent.com). X. Yu, B. Zhang, and J. S. Harris, Jr., are with the Solid State and Photonics Laboratories, Stanford University, Stanford, CA 94305 USA. Digital Object Identifier 10.1109/LPT.2005.860062 signals of the detector include all the spectrum information of the light waves, and transform analysis is used to extract the spectra. For a system using a thin-film photodiode detector, the spectrometer resolution is dependent on the traveling path of the scan mirror following the Fourier transform convolution of the spectra of the sampling window and the standing-wave light fields. In the novel approach we demonstrate in this letter, the normal photodetector is replaced by a thin-film heterojunction bipolar phototransistor (HPT). Different from the normal metal-semi- conductor–metal detectors or p–i–n photodiode detectors whose photocurrents are linearly proportional to the incident light in- tensity, the photocurrent of the HPT has a nonlinear relationship with the light field intensity. Except for the photocurrent gener- ation in the thin base region, the photocurrent is amplified by the transistor benefiting from the emitter-base heterojunction. The internal current gain of an HPT is highly dependent on the device structures, including layer thickness and doping level of the emitter, base, and collector regions as well as the material quality and interface growth condition. Even for a specific HPT device, the internal current gain is not constant, corresponding to different collector currents. The utilization of an HPT as the photodetector in the SWFTIR introduces distortion of the sinusoidal standing-wave light fields. General Fourier transform theory predicts the existence of higher order harmonic wave components on the spectrum of the distorted sinusoidal signals. By making use of the th-order harmonic frequency, we can improve the resolu- tion of the interferometer by times. With ideal signal-to-noise ratio of the spectrum, the resolution of the interferometer is not limited by the travel path of the scan mirror. This configuration is highly desirable for compact system designs without limita- tion of the scanning elements. II. THEORY OF HARMONIC SPECTRAL RESOLUTION IN FOURIER SPECTRUM The sampling window due to the mirror scanning corresponds to a square wave in the time domain and the Fourier spectrum is a sinc function. For a mirror travel length of (in centimeters), the corresponding Fourier spectrum linewidth [full-width at half-maximum (FWHM)] is 1.2 (in centimeters ). The traditional standing-wave Fourier transform interferometer has a spectral resolution of 0.6 (in centimeters ). Based on Fourier transform theory, there will be higher order harmonic in the wave spectrum if the periodical signals deviate from an ideal sinusoidal pattern. So, for sampling a nonsinusoidal periodical signal, a series of order of harmonic waves will show up in the transform spectrum. 1041-1135/$20.00 © 2005 IEEE