1. Field of the Invention
This invention relates to methods of and apparatus for measuring the frequency response of optical detectors. Accordingly, it is a general object of this invention to provide new and improved methods and apparatus therefor.
2. General Background
The following listing of publications may be of interest to the reader. The relevance of the listed items will be apparent as they are referred hereinafter in the specification by their associated superscripts:
[1] H. Blauvelt et al., Fabrication and characterization of GaAs Schottky barrier photodetectors for microwave fiber optic links, Appl. Phys. Lett. 45 pp. 195-196 (1984) PA1 [2] Torbjom Anderson et al., Temporal and frequency response of avalanche photodiodes from noise measurements, Appl. Optics 19 pp. 3496-3499 (1980) PA1 [3] C. A. Burrus et al., Improved very-high-speed packaged InGaAs PIN punchthrough photodiode, Elect. Lett. 21 pp. 262-263 (1985) PA1 [4] L. Piccari and P. Spano, New method for measuring ultrawide frequency response of optical detectors, Elect. Lett. 18 pp. 116-118 (1981) PA1 [5] C. H. Henry, Theory of the linewidth of semiconductor lasers, IEEE JQE 18 pp. 259-264 (1982) PA1 [6] S. Kobayashi et al., Direct frequency modulation in AlGaAs semiconductor lasers, IEEE JQE 18 pp. 582-195 (1982) PA1 [7] E. Eichen et al., Intrinsic lineshape and FM response of modulated semiconductor lasers, Elect. Lett. 21 pp. 849-850 (1985) PA1 [8] J. Schlafer et al., 20 GHz Bandwidth InGaAs Photodetector for Long-Wavelength Microwave Optical Links, Elect. Lett. 21 pp. 469-471 (1985)
As the bandwidth of optical detectors reaches beyond 100 GHz, it becomes increasingly difficult to measure their frequency response. Swept frequency measurements require either an intensity modulated laser or an external modulator, both with a known response greater than that of the detector.sup.1 ; due to the extremely low signal levels obtained during the measurement of the shot noise frequency spectrum, the utilization of amplifiers is required.sup.2 ; time domain measurements requiring picosecond optical pulses are easily affected by sampling and computational errors.sup.3 ; and the beat frequency method requires two narrow linewidth (usually external cavity) semiconductor lasers with excellent temperature control and no optical feedback.sup.4. Swept frequency and shot noise measurements are typically limited in bandwidth, and the other techniques can be experimentally difficult and/or expensive.
When the amplitude modulation portion of the electric field of a semiconductor laser is modulated, by modulating the current a side effect results in that the optical frequency of the electric field is also modulated.sup.5-6.
In the absence of an interferometer, the modulation of current in a semiconductor laser, at a given frequency (such as 100 Hertz, for example), results in a light intensity that is also modulated at the same frequency (100 Hertz, in this example).