Patent No. 6834134 Method and apparatus for generating frequency modulated pulses

 

Patent No. 6834134

Method and apparatus for generating frequency modulated pulses (Brennan, III, et al., Dec 21, 2004)

Abstract

A method and apparatus are provided for generating short (e.g., picosecond) pulses using a 2 section 1553 nm DBR laser without gain switching nor external modulation. The center wavelength of the DBR section is modulated at 0.5 GHz to generate a constant amplitude frequency modulated optical wave Large group velocity dispersion is then applied with a chirped fiber Bragg grating to convert the FM signal to a pulse stream.

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FIELD OF THE INVENTION

The present invention relates generally to modulation of optical carrier signals, and more particularly to the frequency modulation of such pulses.

BACKGROUND OF THE INVENTION

The spectral shaping of optical pulses has been studied extensively, and is the subject of numerous articles, patents, and patent applications. Much of this work has concerned amplitude modulation of laser pulses. U.S. Pat. No. 5,912,999 (Brennan III, et al.) is representative of this technology, as are U.S. Ser. No. 09/401,160, entitled "Method and Apparatus for Arbitrary Spectral Shaping of an Optical pulse", filed Sep. 22, 1999; U.S. Ser. No. 09/161,944, entitled "Long-Length Continuous Phase Bragg Reflectors in Optical Media", filed on Sep. 28, 1998; and U.S. Ser. No. 09/110,495, entitled, "Method for Writing Arbitrary Index Perturbations on a Waveguide", filed Jul. 6, 1998.

Frequency modulation of optical pulses has also been studied to some extent. Thus, researchers have investigated both active and passive mode locking of multiple longitudinal (axial) optical modes of laser cavities. Examples of active mode locking are described in S. E. Harris, R. Targ, Appl. Phys. Lett., 5, 202 (1964), E. O. Ammann, B. J. McMurtry, M. K. Oshman, IEEE JQE, QE-1, 263 (1965), D. J. Kuizenga, A. E. Siegman, IEEE JQE, QE-6, 673 (1970), and R. Nagar, D. Abraham, N. Tessler, A. Fraenkel, G. Eisenstein, E. P. Ippen, U. Koren, G. Raybon, Opt. Lett., 16, 1750 (1991). Examples of passive mode locking are described in L. F. Tiemeijer, P. I Kuindersma, P. J. A. Thijs, G. L. J. Rikken, IEEE JQE, 25, 1385 (1989), and S. R. Chinn, E. A. Swanson, IEEE Phot. Tech. Lett., 5, 969 (1993). However, there have been no reports to date of lasers exhibiting FM operation with a single longitudinal mode.

M. McAdams, E. Peral, D. Provenzano, W. Marshall, and A. Yariv, Appl. Phys. Lett. 71 (7) 879 (Aug. 18, 1997) describes a method for converting frequency modulation to amplitude modulation by transmitting the signal of a semiconductor laser through an optical isolator and into a fiber pigtail comprising various lengths of single-mode non-dispersion shifted fiber and/or an unchirped fiber grating. The reference notes that, in a directly modulated semiconductor laser, a frequency modulation or chirp inevitably accompanies modulation of the amplitude. This work tried to improve the frequency response of a modulated DF laser by frequency modulation of its output.

Some telecommunications applications of lasers require a stable, low-jitter source of ultrashort pulses at typical fiber optics telecommunications wavelengths (approximately 1300 and 1550 nm). Present methods of obtaining short pulses from semiconductor lasers at these wavelengths typically involve gain switching or mode locking. However, gain switching is often plagued by inherent instabilities that arise from the need for the laser to build up from below the lasing threshold for each pulse. Active mode locking can also be unstable because the mode locking frequency must remain tuned to the cavity resonant frequency, which can drift with temperature changes or other environmental effects.

There is thus a need in the art for a method for generating a stable, low jitter source of optical pulses suitable for use in the telecommunications industry. There is also a need in the art for a device suitable for generating such pulses.

These and other needs are met by the present invention, as hereinafter described.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a method and device for generating frequency modified (FM) pulses. In accordance with the method, a short cavity single longitudinal mode laser is employed as a source that can be frequency modulated by rapidly tuning the distributed Bragg Reflector (DBR) section of the laser. This technique produces results similar to FM modelocked pulse sources. However, the source can be modulated at frequencies not synchronous with the cavity resonance.

In another aspect, the present invention relates to a method for generating picosecond pulses at an electronically defined repetition rate without gain switching, modelocking, nor external modulation. In accordance with the method, a 1553 nm DBR laser coupled to a chirped fiber grating is used as a pulse source. The pulse source exhibits stable operation and potentially low timing jitter.

In another aspect, the present invention relates to a picosecond optical pulse source consisting of a frequency modulated semiconductor laser with high modulation depth and a long chirped fiber Bragg grating with large group velocity dispersion. Unlike modelocked lasers, this source has a repetition rate which is not required to be synchronous with the laser cavity resonance, enabling stable operation. Because frequency modulation does not require gain switching, there is potential for very low timing jitter. The FBG also provides potential for higher pulse energies, lower background level, and more efficient use of the total laser energy output.

In yet another aspect, the present invention relates to a method for generating a pulse stream, and to the pulse stream so obtained. In accordance with the method, a sinusoidally varying current is applied to the mirror section of a 2-section distributed Bragg reflector laser, thereby modulating its lasing frequency to generate an frequency modulated optical wave. The modulation rate is arbitrary, as long as it is much lower than the cavity's fundamental resonance. At the laser output, a large group velocity dispersion is applied with a chirped fiber Bragg grating to convert the frequency modulated signal to a pulse stream. The effect of the group velocity dispersion is that the up-chirped portion of the signal is compressed into pulses while the down-chirped portion is further chirped and dispersed into the background. With sinusoidal modulation and linear dispersion, the pulses contain approximately 40% of the total energy.