# Patent

## Title
Multi-ring resonator implementation of optical spectrum reshaper for chirp managed laser technology

## Abstract

There is provided a fiber optic transmission system, comprising:

    
    
        an optical signal source adapted to produce a frequency modulated signal; and
        a multi-ring resonator optical spectrum reshaper (OSR) adapted to convert the frequency modulated signal into a substantially amplitude modulated signal.
    
    


And there is provided a method for transmitting an optical signal through a fiber comprising:

    
    
        producing a frequency modulated signal;
        passing the frequency modulated signal through a multi-ring resonator optical spectrum reshaper (OSR) so as to convert the frequency modulated signal into a substantially amplitude modulated signal; and
        passing the substantially amplitude modulated signal into the fiber.

## Background
REFERENCE TO PENDING PRIOR PATENT APPLICATIONS 
This patent application is:
(i) a continuation-in-part of prior U.S. patent application Ser. No. 10/289,944, filed Nov. 6, 2002 now U.S. Pat. No. 6,963,685 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM
(ii) a continuation-in-part of pending prior U.S. patent application Ser. No. 10/308,522, filed Dec. 3, 2002 by Daniel Mahgerefteh et al. for HIGH-SPEED TRANSMISSION SYSTEM COMPRISING A COUPLED MULTI-CAVITY OPTICAL SCANNER;
(iii) a continuation-in-part of prior U.S. patent application Ser. No. 10/680,607, filed Oct. 6, 2003 by now U.S. Pat. No. 7,054,538 Daniel Mahgerefteh et al. for FLAT DISPERSION FREQUENCY DISCRIMINATOR (FDFD);
(iv) claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/530,479, filed Dec. 17, 2003 by Daniel Mahgerefteh et al. for OPTICAL TELECOMMUNICATION SYSTEM;
(v) claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/548,230, filed Feb. 27, 2004 by Yasuhiro Matsui et al. for OPTICAL SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING ELEMENT;
(vi) claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/569,769, filed May 10, 2004 by Daniel Mahgerefteh et al. for FLAT CHIRP INDUCED BY AN OPTICAL FILTER EDGE; and
(vii) claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/629,741, filed Nov. 19, 2004 by Yasuhiro Matsui et al. for OPTICAL SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING ELEMENT.
The seven above-identified patent applications are hereby incorporated herein by reference.
This patent application is also a continuation-in-part of pending prior U.S. patent application Ser. No. 10/933,081, filed Sep. 2, 2004 by Daniel Mahgerefteh et al. for METHOD AND APPARATUS FOR TRANSMITTING A SIGNAL USING THERMAL CHIRP MANAGEMENT OF A DIRECTLY MODULATED TRANSMITTER.


FIELD OF THE INVENTION 
This invention relates to signal transmissions in general, and more particularly to the transmission of optical signals.
BACKGROUND OF THE INVENTION 
A system for long-reach lightwave data transmission through optical fiber is disclosed in U.S. patent application Ser. No. 10/289,944, filed Nov. 6, 2002 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM, which patent application is hereby incorporated herein by reference. Azna LLC of Wilmington, Mass. sometimes refers to the transmitter apparatus of this patent application as a Chirp Managed Laser (CMLTM). In this system, a frequency modulated (FM) source is followed by an optical discriminator, also sometimes referred to as optical spectrum reshaper (OSR), which converts frequency modulation into a substantially amplitude modulated (AM) signal and partially compensates for the dispersion in the transmission fiber.
The optical spectrum reshaper (OSR) can be formed by any optical element that has a wavelength-dependent transmission function. The OSR can be adapted to convert frequency modulation to amplitude modulation. Significantly, the OSR can also be adapted to convert amplitude modulation to frequency modulation. The means and advantages of converting amplitude modulation to frequency modulation with an OSR has been described U.S. Provisional Patent Application Ser. No. 60/569,769, filed May 10, 2004 by Daniel Mahgerefteh et al. for FLAT CHIRP INDUCED BY AN OPTICAL FILTER EDGE which patent application is hereby incorporated herein by reference.
The phase imparted by the OSR may also be used to compensate for at least a portion of the dispersion of the transmission fiber, as disclosed in U.S. patent application Ser. No. 10/289,944, filed Nov. 6, 2002 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM.
A variety of optical spectrum reshapers (OSRs), sometimes referred to as optical discriminators in one or more of the above-identified patent applications, are disclosed in the aforementioned U.S. patent application Ser. No. 10/289,944.
Semiconductor material growth make it possible to integrate various optical components, leading to the smaller size, ease of mass production and low manufacturing cost that the electronics industry now enjoys. This is one goal of planar light wave technology, which seeks to replace bulk optical elements used in a transmitter, for example, by waveguide-based devices.
It is, therefore, an object of the present invention to make a CMLTM laser source which can be integrated. The CMLTM laser source comprises a frequency modulated source such as a distributed feedback laser chip (DFB) and an OSR.
SUMMARY OF THE INVENTION 
These and other objects of the present invention are addressed by a novel multi-ring resonator implementation of optical spectrum reshaper for chirp managed laser technology.
In one form of the invention, there is provided a fiber optic transmission system, comprising:
an optical signal source adapted to produce a frequency modulated signal; and
a multi-ring resonator optical spectrum reshaper (OSR) adapted to convert the frequency modulated signal into a substantially amplitude modulated signal.
In another form of the invention, there is provided a method for transmitting an optical signal through a fiber comprising:
producing a frequency modulated signal;
passing the frequency modulated signal through a multi-ring resonator optical spectrum reshaper (OSR) so as to convert the frequency modulated signal into a substantially amplitude modulated signal; and
passing the substantially amplitude modulated signal into the fiber.



BRIEF DESCRIPTION OF THE DRAWINGS 
These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:
FIG. 1 is schematic diagram showing a micro-ring resonator coupled to straight waveguide sections;
FIG. 2 shows a multi-ring resonator geometry which may be used for an optical spectrum reshaper (OSR);
FIG. 3 shows typical spectral response from a 3-ring resonator configuration; and
FIG. 4 defines the peak slope of the transmission profile for the optical spectrum reshaper (OSR).



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the present invention, the spectral characteristics of the OSR can be achieved using coupled micro-ring resonators, which are waveguide devices capable of being integrated. The micro-ring resonators generally comprise small (e.g., 4–25 μm radii) waveguide rings that are coupled to straight waveguide sections, as shown in FIG. 1.
A single ring resonator has a wavelength dependent transmission profile similar to a Fabry-Perot filter, or a single cavity etalon.
The micro-ring resonator functions as follows.
Input to the straight section Ein couples partly into the micro-ring, while a fraction, ET, passes to the straight section on the other side of the micro-ring. Light coupled into the micro-ring, ER1, travels around the ring and couples back into the straight section, having picked up a phase according to ηER exp(iφ). Here η is a fraction depending on the coupling and the loss, and φ is the phase shift caused by traveling around the ring. With each round trip around the micro-ring, part of the light is phase shifted and added to the through field, generating fields ER2, ER3, . . . . The output electric field at the end of the straight section is the interference between the through field ET and the multiplicity of waves, ER1, ER2, ER3, . . . that have traveled 1, 2, 3, . . . n times around the micro-ring. When the interference is constructive, the output field has a peak in its transmission spectrum. The phase difference is determined by the size and effective refractive index of the micro-ring. The part of the optical signal which is not transmitted at the “through port” is rejected at the “drop port” of the micro-ring resonator apparatus.
A micro-ring resonator with multiple micro-rings can have a transmission profile that is similar to a multi-cavity filter, as described in U.S. patent application Ser. No. 10/308,522, filed Dec. 3, 2002 by Daniel Mahgerefteh et al. for HIGH-SPEED TRANSMISSION SYSTEM COMPRISING A COUPLED MULTI-CAVITY OPTICAL SCANNER, which patent application is hereby incorporated herein by reference.
Significantly, the bandwidth and slope of the transmission profile of the multi-ring cavity OSR can be designed by choosing the number, radius, and coupling coefficient of the micro-rings. Recent technical developments have shown that coupled ring resonators can be implemented in integrated optics form using both dielectric-based (i.e., glass-based) and semiconductor-based material systems, as described, for example, by J. V. Hryniewicz et al., IEEE Photonics Technology Letters, vol. 12, No. 3, p. 320 (2000), which publication is hereby incorporated herein by reference. Furthermore, ring resonator geometries have been demonstrated using both planar and vertical coupling schemes. For example, a 3-ring resonator, flat-topped, wavelength division multiplexed (WDM) ring resonator device was demonstrated by Little Optics (Advances in Microring Resonators, Brent E. Little, LittleOptics, Inc, Integrated Photonics Research Conference, 2003), which publication is hereby incorporated herein by reference. Similar spectral responses have been demonstrated for ring resonator architectures implemented in a variety of material systems including Silicon, InP, GaAs, glass, Silica, etc.
FIG. 2 shows a preferred implementation of a multi-ring resonator geometry that may be used for CMLTM technology. The material system has a refractive index close to n=1.6, such that a ring diameter of 1.1 mm results in a periodicity of 50 GHz. The optical length of the ring is tailored to produce a periodicity of 50 GHz so as to allow the spectral response to be used for wavelength locking to the 50 GHz-spaced ITU grid, which is the common telecommunications standard, and also so as to allow the implementation of a tunable version of the CMLTM system. The couplings between the input and output waveguides and the rings, and the couplings between adjacent waveguide rings, can be controlled to produce the desired spectral response.
FIG. 3 is a graph plotting transmission as a function of optical frequency for the OSR which may be formed by the multi-ring resonator. In a preferred embodiment of the present invention, shown in FIG. 3, the operating point of the input frequency modulated signal is at the edge of the transmission peak of the OSR. Note that in the standard usage of the multi-ring filter, the input wavelength is centered on the transmission peak of the multi-ring resonator so as to reduce loss. In the CMLTM application of the present invention, the edge of the transmission is used to convert the frequency modulated signal to a substantially amplitude modulated signal. The transmission edge of the multi-ring resonator can also be adapted to convert amplitude modulation to appropriate frequency modulation so as to generate flat chirp and thereby increase the transmission distance of the resulting output signal in fiber with dispersion, as described in U.S. Provisional Patent Application Ser. No. 60/569,769, filed May 10, 2004 by Daniel Mahgerefteh et al. for FLAT CHIRP INDUCED BY AN OPTICAL FILTER EDGE, which patent application is hereby incorporated herein by reference.
FIG. 4 defines the peak slope of the transmission profile for the OSR. The peak slope of the transmission peak of the ring resonator OSR is preferably in the range of about 2 dB/GHz to about 3 dB/GHz for a system having a 10 Gb/s bit rate of operation. The bandwidth of the ring resonator filter is preferably about 0.8 times to about 1.2 times the bit rate of operation.
In an embodiment of the present invention, the design parameters of the multi-ring resonator, such as coupling and ring radii, can be varied so as to create a transmission profile which is substantially Gaussian. As described in U.S. patent application Ser. No. 10/680,607, filed Oct. 6, 2003 by Daniel Mahgerefteh et al. for FLAT DISPERSION FREQUENCY DISCRIMINATOR (FDFD), which patent application is hereby incorporated herein by reference, a substantially Gaussian shape can be beneficial. In another embodiment of the present invention, the spectral shape is made to be nearly flat-topped for a low bit rate (e.g., 2.5 Gb/s) application, with a slope near the top of the filter of <0.2 dB/GHz.
The aforementioned CMLTM technology can be applied at a 10 Gb/s transmission bit rate to distances of greater than 200 km of standard single mode fiber having a net dispersion of 3200 ps/nm, as described in U.S. Provisional Patent Application Ser. No. 60/548,230, filed Feb. 27, 2004 by Yasuhiro Matsui et al. for OPTICAL SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING ELEMENT, which patent application is hereby incorporated herein by reference. In this case, the requirements for the multi-ring resonator OSR transmission is different, as disclosed in U.S. Provisional Patent Application Ser. No. 60/629,741, filed Nov. 19, 2004 by Yasuhiro Matsui et al. for OPTICAL SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING ELEMENT. More particularly, in this case, a key parameter of the OSR is the slope of its logarithmic slope, defined as the ratio of the peak slope in dB/GHz to the frequency offset of the peak slope from the transmission peak in GHz, as shown in FIG. 4. For example, for a 10 Gb/s transmitter, a low bit error rate after transmission is obtained if the slope of the slope is approximately in the range of 0.38 dB/GHz2 to 0.6 dB/GHz2. In addition, the slope of the OSR near the center of the transmission should be approximately linear. Deviations from linearity introduce distortions in the resulting output eye diagram and increased bit error rate. A linear slope corresponds to a round-topped shaped filter. So, for example, a flat-top filter, which has a near zero slope near the center, is generally not desirable for this long reach application. The 3 dB bandwidth of the bandpass OSR should be in the range of 65% to 90% of the bit rate for this long reach application.
A wavelength locking circuit for the CMLTM system is described in U.S. patent application Ser. No. 10/680,607, filed Oct. 6, 2003 by Daniel Mahgerefteh et al. for FLAT DISPERSION FREQUENCY DISCRIMINATOR (FDFD), which patent application is hereby incorporated herein by reference. In this arrangement, the optical power of the source, PS, and optical power transmitted through the spectrum reshaper, PT, are measured using taps and the ratio PT/PS held constant. In an embodiment of the present invention, a tap splitter on the input of the straight section couples part of the input light so as to measure input power, PS, and a second tap at the output waveguide can be added to couple part of the output power and measure PT. Photodiodes placed to receive the outputs of the two taps, respectively, are then used as part of the wavelength locking circuit such as that described in the aforementioned U.S. patent application Ser. No. 10/680,607. The wavelength of the source is kept locked to the transmission edge of the multi-ring resonator by adjusting the wavelength of the source in order to keep the ratio PT/PS constant.
In one preferred implementation, the multi-ring resonator geometry is integrated with the source, to gain the benefit of wafer level fabrication whereby to obtain lower chip costs and increase yield. A key advantage of the multi-ring resonator in the CMLTM application is that the rejected light does not reflect back into the source, thus eliminating the need for an optical isolator. This makes it possible to integrate the source, such as a distributed feedback laser (DFB) with the multi-ring resonator OSR. Further advantages of an integrated version of the optical discriminator include small size, fast thermal response time and higher wavelength sensitivity (for semiconductor-based implementations). In particular, the possibility of integrating the discriminator function together with the DFB laser on a common chip exists if InP is used as the material system. In this case, the relative spectral positions of the discriminator and the laser wavelength can track with temperature and allow a simple control algorithm to be used. It is, therefore, an embodiment of the present invention to integrate a frequency modulated source, such as a DFB laser, with the multi-ring cavity OSR.
In another embodiment of the present invention, a DFB array (comprising a plurality of DFB lasers, each having a different wavelength of operation) is integrated on the same chip with a plurality of multi-ring resonators so as to form a multi-wavelength CMLTM source. These separate wavelengths are then combined by another multiplexer multi-ring resonator.
It will be appreciated that still further embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure. It is to be understood that the present invention is by no means limited to the particular constructions herein disclosed and/or shown in the drawings, but also comprises any modifications or equivalents within the scope of the invention.

## Claims

1. A fiber optic transmission system comprising:
an optical signal source adapted to produce a frequency modulated signal; and
a multi-ring resonator optical spectrum reshaper (OSR) adapted to convert the frequency modulated signal into a substantially amplitude modulated signal.

2. A fiber optic system according to claim 1 wherein the operating point of the frequency modulated signal is near the edge of the transmission peak of the multi-ring resonator.

3. A fiber optic system according to claim 1 wherein the frequency modulated source is a distributed feedback (DFB) laser.

4. A fiber optic system according to claim 3 wherein the DFB laser is integrated with the multi-ring resonator on the same substrate.

5. A fiber optic system according to claim 1 wherein the frequency modulated source is a DFB laser array.

6. A fiber optic transmission system comprising:
an optical signal source adapted to produce a frequency modulated signal; and
a multi-ring resonator optical spectrum reshaper (OSR) adapted to convert the frequency modulated signal into a substantially amplitude modulated signal,
wherein the multi-ring resonator OSR comprises coupled straight sections and ring waveguides.

7. A fiber optic system according to claim 6 wherein the number, the radii of the rings and the coupling between the rings is adjusted to produce a desired transmission profile.

8. A fiber optic system according to claim 7 wherein the transmission function is a periodic function with a periodicity equal to an integer multiple of 50 GHz.

9. A fiber optic system according to claim 7 wherein the transmission spectrum of the multi-ring resonator is substantially Gaussian near each transmission peak.

10. A fiber optic system according to claim 9 wherein the 3 dB bandwidth of the transmission spectrum is approximately 0.8 times to approximately 1.2 times the bit rate of operation.

11. A fiber optic system according to claim 9 wherein the peak slope of the transmission profile is approximately 2 dB/GHz to approximately 3 dB/GHz.

12. A fiber optic system according to claim 7 wherein the transmission spectrum of the multi-ring resonator is substantially flat-topped near each transmission peak.

13. A fiber optic system according to claim 12 wherein the slope of the transmission function is <0.2 dB/GHz near the transmission peak.

14. A fiber optic transmission system comprising:
an optical signal source adapted to produce a frequency modulated signal; and
a multi-ring resonator optical spectrum reshaper (OSR) adapted to convert the frequency modulated signal into a substantially amplitude modulated signal,
wherein the slope of the slope of the transmission of the multi-ring resonator, measured in dB, is approximately 0.36 dB/GHz2 to approximately 0.6 dB/GHz2.

15. A fiber optic system according to claim 14 wherein the 3 dB bandwidth of the transmission approximately is 65% to approximately 90% of the bit rate.

16. A fiber optic transmission system comprising:
an optical signal source adapted to produce a frequency modulated signal; and
a multi-ring resonator optical spectrum reshaper (OSR) adapted to convert the frequency modulated signal into a substantially amplitude modulated signal,
further including a wavelength locking circuit adapted to wavelength lock the frequency modulated input signal to the multi-ring resonator by comparing the optical power of the input signals to the multi-ring resonator with the optical power of the transmitted signals from multi-ring resonator at the through port and then adjusting the optical signal source to keep the ratio of the input optical power and the transmitted optical power substantially constant.

17. A fiber optic system according to claim 16 further including a first tap at the input to the multi-ring resonator, a first photodiode to receive the output of the first tap, a second tap at the output through port of the multi-ring resonator, and a second photodiode to receive the output of the second tap.

18. A fiber optic system according to claim 17 wherein the first and second taps are integrated on the same substrate as the multi-ring resonator.

19. A fiber optic transmission system comprising:
an optical signal source adapted to produce a frequency modulated signal; and
a multi-ring resonator optical spectrum reshaper (OSR) adapted to convert the frequency modulated signal into a substantially amplitude modulated signal,
further including a wavelength locking circuit adapted to wavelength lock the frequency modulated input signal to the multi-ring resonator by comparing the optical power of the input signals to the multi-ring resonator with the optical power of the rejected signals from multi-ring resonator at the drop port and then adjusting the optical signal source to keep the ratio of the input optical power and the rejected optical power substantially constant.

20. A fiber optic system according to claim 19 further including a first tap at the input to the multi-ring resonator, a first photodiode to receive the output of the first tap, a second tap at the drop port of the multi-ring resonator, and a second photodiode to receive the output of the second tap.

21. A fiber optic system according to claim 20 wherein the first and second taps are integrated on the same substrate as the multi-ring resonator.

22. A method for transmitting an optical signal through a fiber comprising:
producing a frequency modulated signal;
passing the frequency modulated signal through a multi-ring resonator optical spectrum reshaper (OSR) so as to convert the frequency modulated signal into a substantially amplitude modulated signal; and
passing the substantially amplitude modulated signal into the fiber.