Optical frequency shifter and transmission system

• US 6,259,836 B1
• Filed: 05/14/1998
• Issued: 07/10/2001
• Est. Priority Date: 05/14/1998
• Status: Expired due to Fees

• Current Assignee: Telecommunications Research Laboratories
• Original Assignee: Telecommunications Research Laboratories
• Inventors: Dodds, David E.
• Primary Examiner(s): Bovernick, Rodney
• Assistant Examiner(s): Stahl, Michael J
• Application Number:
  US09/079,133
• Time in Patent Office: 1,153 Days
• Field of Search: 385/1-3, 385/24, 385/27, 385/39, 359/124, 359/127, 359/326, 359/190, 359/174, 359/175
• US Class Current: 385/24
• CPC Class Codes: G02F 2/02 Frequency-changing of light, e.g. by quantum counters
• CPC Class Codes: H04J 14/02 Wavelength-division multiplex systems
• CPC Class Codes: H04J 14/0305 in end terminals

Abstract

An optical transmission system has an input light guide and N multipath modulators operated as frequency shifters connected in cascade to receive light from the input light guide and output light along N output light guides where N is 1 or more, wherein the light carried by each ith output light guide, where i=1, . . . , N, is shifted in frequency from light carried by the input light guide by iω_m, and where each set of multipath modulators contributes a frequency translation of ω_m and where the light from each output light guide is modulated and combined with all other modulated output light to form a multiwavelength transmission system. A receiver equipped with similar frequency shifters is configured to generate a number of optical reference signals of frequencies ω₀+iω_m, each reference sign to be added as a separate portion of the received signal so as to perform self-homodyne detection of the information signal associated with each respective modulated frequency.

Claims

1. A multiple optical carrier generator comprising a cascaded chain of N optical frequency shifters, the chain of optical frequency shifters including a final optical frequency shifter, each optical frequency shifter comprising:

an input light guide;

a first set of modulators in parallel and each connected to receive light from the input light guide via a first splitter;

an output light guide connected to receive light from the first set of modulators through an optical combiner, each output light guide other than the output light guide from the final optical frequency shifter forming an input light guide to a subsequent optical frequency shifter in the chain of optical frequency shifters;

plural voltage sources, each modulator of the first set of modulators being connected to receive a modulating voltage from a respective one of the voltage sources;

the voltage sources having output voltages characterized such that modulated light appearing on the output light guide of each optical frequency shifter is translated in frequency by a frequency ω

₁ from light input to the input light guide of the optical frequency shifter, whereby the chain of optical frequency shifters produces a set of N multiple optical carriers ω

₀+iω

₁ where i=1 . . . N;

and the multiple optical carrier generator further comprising;

a respective one of a set of compensating amplifiers connected at the output of each optical frequency shifter so as to maintain a nearly constant amplitude of light in the chain of optical frequency shifters; and

optical taps at each amplifier to provide an output of each optical carrier for use in transmitters and receivers of a wavelength division multiplexed optical communication system.

2. The multiple optical carrier generator of claim 1 in which each set of modulators is comprised of two Mach Zehnder modulators in parallel.

3. The multiple optical carrier generator of claim 1 in which each set of modulators comprises a modulator having three or more modulator paths in parallel.

4. The multiple optical carrier generator of claim 1 in which each set of modulators comprises:

a first amplitude modulator and phase modulator in a first path for generating a single sideband signal including a carrier signal;

a second path with modulators configured to generate a signal with equal amplitude and opposite phase to carrier signal appearing in the first path; and

a combiner for combining signals on the first and second paths.

5. The multiple optical carrier generator of claim 1 further comprising:

a second set of modulators, each modulator of the second set of modulators being connected to a corresponding one of the optical taps for modulating an information signal onto the light carried by each respective optical tap.

6. An optical transmission system comprising:

(a) a transmitter incorporating a multiple optical carrier generator comprising a cascaded chain of N optical frequency shifters, the chain of optical frequency shifters including a final optical frequency shifter, each optical frequency shifter comprising;

an input light guide;

a first set of modulators in parallel and each connected to receive light from the input light guide via a first splitter;

an output light guide connected to receive light from the first set of modulators through an optical combiner, each output light guide other than the output light guide from the final optical frequency shifter forming an input light guide to a subsequent optical frequency shifter in the chain of optical frequency shifters;

plural voltage sources, each modulator of the first set of modulators being connected to receive a modulating voltage from a respective one of the voltage sources;

the voltage sources having output voltages characterized such that modulated light appearing on the output light guide of each optical frequency shifter is translated in frequency by a frequency ω

₁ from light input to the input light guide of the optical frequency shifter, whereby the chain of optical frequency shifters produces a set of N multiple optical carriers ω

₀+iω

₁ where i=1 . . . N;

and the multiple optical carrier generator further comprising;

a respective one of a set of compensating amplifiers connected at the output of each optical frequency shifter so as to maintain a nearly constant amplitude of light in the chain of optical frequency shifters;

optical taps at each amplifier to provide an output of each optical carrier for use in transmitters and receivers of a wavelength division multiplexed optical communication system;

AND

a second set of modulators, each modulator of the second set of modulators being connected to a corresponding one of the optical taps for modulating an information signal onto the light carried by each respective optical tap

(b) a receiver connected to an optical fiber; and

(c) a combiner in the transmitter, the combiner being connected to the optical fiber and to each optical tap, for combining modulated light from each optical tap to form multiple modulated optical carriers to be transmitted over the optical fiber to the receiver.

7. The optical transmission system of claim 6 in which the optical receiver comprises:

a synthesizer incorporating a multiple optical carrier generator for generating reference signals corresponding to the frequencies generated by the multiple optical carrier generator of the transmitter;

plural mixers connected to the synthesizer for adding the reference signals to light received from the transmitter via the optical fiber; and

plural detectors, each of the plural detectors being connected at the output of a respective mixer, for detecting information signals modulated onto a corresponding one of the multiple optical carriers.

8. The optical transmission system of claim 6 in which each modulator for modulating respective information signals onto the respective optical carrier signals is configured such that the modulated optical carrier signal contains sideband components and substantially no component at the frequency of the optical carrier.

9. The optical transmission system of claim 7 in which each modulator for modulating respective information signals onto the respective optical carrier signals is configured such that the modulated optical carrier signal contains sideband components and substantially no component at the frequency of the optical carrier.