All-pass optical filters

Details All-pass optical filters All-pass optical filters

1998-10-30

2001-09-11

Font, Frank G. (Department: 2877)

Optical waveguides

With optical coupler

Input/output coupler

C385S024000, C385S027000, C359S199200

Reexamination Certificate

active

06289151

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to optical communication systems, and more particularly, to optical filters.
DESCRIPTION OF THE RELATED ART
Optical communication systems typically include a variety of devices (e. g., light sources, photodetectors, switches, optical fibers, modulators, amplifiers, and filters). For example, in the optical communication system
1
shown in
FIG. 1
, a light source
2
generates an optical signal
3
. The optical signal
3
comprises a series of light pulses. The light pulses are transmitted from the light source
2
to a detector
5
. Typically, an optical fiber
4
transmits the light pulses from the light source
2
to the detector
5
. The optical fiber
4
has amplifiers (not shown) and filters (not shown) positioned along its length. The amplifiers and filters propagate the light pulses along the length of the optical fiber
4
from the light source
2
to the detector
5
.
Optical communication systems are useful for transmitting optical signals over long distances at high speeds. For example, optical signals are routinely transmitted distances greater than about 60 kilometers at transmission speeds exceeding 1 Gbit/s (Gigabit/second).
As shown on the graph of
FIG. 2A
, an optical pulse
10
typically comprises a packet of waves, wherein each wave in the packet, denoted as
15
, is within a frequency bandwidth of &Dgr;ƒ. Additionally, each wave
15
in the packet is characterized by a plurality of different frequencies as well as a plurality of different amplitudes. An optical device or component (e. g., amplifiers, filters, and fibers) has an amplitude response and a phase response. The amplitude response describes the attenuation of each frequency in the optical pulse after transmission through the optical device relative to their attenuation before transmission through the optical device. The phase response determines the time delay, denoted as
17
, for each frequency
15
in the packet of waves.
Many optical devices or components used for transmitting optical pulses apply a nonlinear phase response to the optical pulse. The nonlinear phase response changes the separation time between each frequency
15
of the packet of waves, causing each frequency
15
to be delayed for a different length of time. When each frequency
15
of the packet of waves is delayed for a different length of time, the optical pulse
10
output from such device or component is broadened and/or distorted, as shown on the graph of FIG.
2
B. Broadening the optical pulse is undesirable because, depending on the time between optical pulses, the leading and trailing edges of the broadened pulse, denoted as
25
, potentially interfere with the trailing edge of a previous optical pulse or the leading edge of a subsequent optical pulse, causing transmission errors. Devices or components in optical communication systems which broaden optical pulses are termed dispersive devices.
The dispersion of optical signals caused by the dispersive devices can be reduced with a dispersion compensating element. The term dispersion as used in this disclosure refers to the first and higher order derivatives of the group delay that are applied to the optical signal. The term group delay refers to the slope of the phase response at each frequency in the packet of waves. The dispersion compensating element applies a second dispersion to the optical signal which is the negative of the dispersion that was caused by the dispersive device. The second dispersion is additive with the dispersion applied by the dispersive device, so the net dispersion of the optical signal is about zero.
Dispersion compensating fibers and chirped fiber Bragg gratings are examples of fiber dispersion compensating elements. However, dispersion compensating fibers are lossy (~5-10 dB). Lossy fibers are undesirable because they potentially reduce the optical power of signals transmitted along their length. Many chirped fiber Bragg gratings typically only compensate for quadratic dispersion, limiting their utility to systems with quadratic dispersion. Also, chirped fiber Bragg gratings require a circulator for separating dispersion compensated optical signals from non-compensated optical signals. Additionally, chirped fiber Bragg gratings are long devices, making them expensive to integrate into optical communication systems.
Another dispersion compensating element, a dispersion equalizer, is described in Takiguchi, et al., “Variable Group-Delay Dispersion Equalizer Using Lattice-Form Programmable Optical Filter on Planar Lightwave Circuit”,
IEEE J. of Quant. Elect
., Vol. 2, No. 2, June 1996, pp. 270-276. The Takiguchi et al. dispersion equalizer is a filter which includes i+1 symmetrical Mach-Zehnder interferometers (MZIs) interleaved with i asymmetrical MZIs, where i is an integer. The term “symmetrical” as used in this disclosure means that the lengths of the two waveguide arms of the MZIs are the same, while the term “asymmetrical” means that the lengths of the two waveguide arms are different. While the Takiguchi et al. dispersion equalizers compensate for dispersion, the amplitude and phase responses are not independent. The result is that a frequency-dependent loss, introduced by the filter, potentially reduces the useable pass-band width of the filter. Additionally, the Takiguchi et al. dispersion equalizer is expensive and difficult to fabricate since a large number of symmetrical MZIs and asymmetrical MZIs are needed for dispersion compensation.
Moslehi et al. (U.S. Pat. No. 4,768,850) also describes a dispersion compensating filter. The dispersion compensating filter is a cascaded fiber optic lattice filter. The cascaded fiber optic filter uses a cascade of recursive and non-recursive fiber optic lattice filters to compensate for dispersion. However, the single-stage cascaded fiber optic lattice filters of Moslehi et al. are lossy (~20 dB). Since the Moslehi et al. devices are lossy, signal amplification is required for providing optical signals having adequate signal strengths for transmission along the optical fiber.
Dilwali, S. et al., “Pulse Response of a Fiber Dispersion Equalizing Scheme Based on an Optical Resonator”,
IEEE Phon. Tech. Lett
., Vol. 4, No. 8, pp 942-944 (1992) proposes the use of a single-stage fiber ring structure as a dispersion equalizer in optical fibers. However, dispersion compensation using a single stage fiber ring structure affects the useable pass-band of the filter, reducing it.
Li, K. D. et al., “Broadband Cubic-Phase Compensation with Resonant Gires-Tournois Interferometers”,
Optics Lett
., Vol. 14, No. 9, pp 450-452 (May 1989) discloses the use of Gires-Tournois interferometers (GTIs) for the dispersion compensation of ultrashort laser pulses. Each GTI includes two reflectors which are separated one from the other by a fixed distance. One reflector has a reflectivity of 100% and the other reflector has a reflectivity less than 100%. However, the GTI arrangement only provides moderate dispersion compensation to the ultrashort laser pulses, limiting the ability for transmitting optical signals at high bit rates.
Additionally, optical filters which apply time delays are useful for synchronizing bit streams of optical signals. For example, optical communication systems utilizing time division multiplexed (TDM) techniques (see Hall, K. L. et al., “All-Optical Storage of a 1.25 kbit Packet at 10 Gb/s”,
IEEE Phon. Tech. Lett
., Vol. 7, No. 9, pp. 1093-1095 (September 1995), require the synchronization of bit streams of optical signals to delay the propagation of some optical signals in time.
SUMMARY OF THE INVENTION
The present invention is directed to an all-pass optical filter. The all-pass optical filter reduces the dispersion of optical pulses transmitted therethrough. The all-pass optical filter reduces the dispersion of optical pulses by applying a desired phase response to optical pulses transmitted therethrough.
The desired phase response applies a frequency-dependent time delay to each frequency of each wave in the packet

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