# Patent

## Title
Low-cost WDM source with an incoherent light injected Fabry-Perot laser diode

## Abstract

The present invention discloses a low-cost light source for optical transmission systems and optical networks based on wavelength-division multiplexing (WDM) technology. A light source in accordance with the present invention is implemented by externally injecting a narrow-band incoherent light into a Fabry-Perot laser diode (F-P LD). After injection of narrow-band incoherent light, the output of F-P LD becomes wavelength-selective rather than multi-mode and the output wavelength of F-P LD coincide with the peak wavelength of the injected incoherent light.
Multi-channel WDM light sources according to the present invention can be implemented using a single broadband incoherent light source and plurality of F-P LDs. An optical transmission system for upstream signal transmission in an passive optical network using the light source according the present invention is also disclosed.

## Background
CROSS-REFERENCED TO RELATED APPLICATIONS
The present application is a divisional application of Ser. No. 11/493,519, filed Jul. 27, 2006, which is a continuation of patent application Ser. No. 09/732,773, filed Dec. 11, 2000, which claims priority to Republic of Korea patent application number 1999-59923, filed Dec. 21, 1999.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light source for optical transmission systems and optical networks based on the wavelength-division multiplexing (WDM) technology, and more particularly to a light source employing a Fabry-Perot laser diode (F-P LD) whose output wavelength is locked by an externally injected incoherent light.
The present invention also relates to WDM transmission systems and WDM passive optical networks using the above-described light sources.
2. Description of the Related Art
Recently, WDM transmission systems are widely deployed to meet the ever-growing bandwidth demands incurred by the explosion of the data traffic. In particular, WDM transmission systems begin to be deployed in metropolitan networks and access networks to accommodate wide-band services such as an electronic commerce, a cable TV, a video conference, and son on.
FIG. 1 shows a configuration of conventional WDM transmission system.
The source node is equipped with multiple transmitters (TXs) with different wavelengths (λ1 ̃λN) and a N×1 multiplexer (MUX) and the destination node is equipped with an 1×N demultiplexer (DMUX) and multiple receivers (RXs). The source node and the destination node are connected through an single strand of optical link composed of optical fibers and optical amplifiers.
In WDM transmission systems described above, communication channels between the source node and the destination node are distinguished one another by their wavelengths. Thus, a unique wavelength is allocated each transmitter-receiver pair. The light, source of transmitter must have the unique wavelength with long-term stability and a large side mode suppression ratio (SMSR) to minimize the interference between neighboring channels. In addition, it is desirable that the light source provides a sufficient output power and has a narrow spectral width.
A representative light source which satisfies the requirements mentioned above is a distributed feedback laser diode (DFB LD). However, since a distributed feedback laser diode is expensive, incoherent light sources are usually used in an access network in which the main concern is the economical competitiveness
The incoherent light sources, such as a light emitting diode (LED), a super-luminescent diode (SLD), and an optical fiber amplifier generating amplified spontaneous emission (ASE), have been used in WDM transmission systems through a spectrum-slicing application. The LED can be fabricated at low cost and modulated directly. However, the output power of LED is not sufficient to accommodate many channels through a spectrum-slicing application. The SLD is costly although it can provide much higher output power than the LED. The optical fiber amplifier can provide a strong incoherent light, ASE, but it requires expensive external modulators.
The F-P LD can provide much higher output power than the LED at the comparable cost with the LED. However, its output is multi-mode and the output power of each mode fluctuates randomly with the time due to the mode hopping and the mode partitioning. Therefore, it has been used in optical transmission systems based on time-division multiplexing technology (TDM) rather than WDM technology. Its application wavelength region was also limited near the zero dispersion wavelength of the optical fiber.
SUMMARY OF THE INVENTION
The objective of the present invention is to provide a low-cost light source for WDM application. The light source according to the present invention is implemented by externally injecting a narrow-band incoherent light into a F-P LD. Its output is wavelength-locked by the externally injected light and thus becomes wavelength-selective.
The other objective of the present invention is to provide WDM transmission systems and WDM passive optical networks employing the light source according to the present invention. The multiple sliced incoherent lights generated from a single broadband incoherent light source are injected into multiple F-P LDs simultaneously to produce multi-channel WDM light sources.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a configuration of conventional WDM transmission system.
FIG. 2 shows the schematic diagram of the light source according to the present invention.
FIG. 3 is a schematic diagram of multi-channel WDM light sources in accordance with the present invention.
FIG. 4a and FIG. 4b show schematically the optical transmission systems for upstream signal transmission in passive optical networks employing the light source in accordance with an embodiment of the present invention.
FIG. 5 shows the experimental set-up to demonstrate the feasibility of the light source in accordance with the present invention.
FIG. 6 shows (a) the output spectrum of the F-P LD without external light injection and (b) the spectrum of the narrow-band ASE to be injected into the F-P LD.
FIG. 7 shows the measured output spectra of the F-P LD after injection of a narrow-band ASE when the injection ASE power were (a) −2 dBm and (b) 2 dBm, respectively.
FIG. 8 shows the measured side-mode-suppression-ratio (SMSR) of the light source in accordance with the present invention.
FIG. 9 shows the measured output spectra of the light source in accordance with the present invention for different bias currents.
FIG. 10 shows the measured the extinction ratio of the light source in accordance with the present invention.
FIG. 11 shows the measured output spectra of the light source in accordance with the present invention when a polarizer and a polarization controller were further used.
FIG. 12 shows the measured bit error rate.



DESCRIPTION OF THE NUMERICS ON THE MAIN PARTS OF THE DRAWINGS


    
    
        TX: a transmitter
        RX: a receiver
        MUX: a multiplexer
        DMUX: a demultiplexer
        ILS: an incoherent light source
        TF: a tunable optical filter
        CIR: an optical circulator
        Pol: a polarizer
        PC: a polarization controller
        F-P LD: a Fabry-Perot laser diode
        ILS: an incoherent light source
        BPF: a band pass filter
        (D)MUX: (de)MUX
        ASE source: an ASE source
        WGR: a waveguide grating router
        AMP1, AMP2: an optical amplifier
        Att.1, Att.2: an optical variable attenuator
        PZF: a polarizing fiber
        SMF: a conventional single mode fiber
        PM: an power meter
    
    


DETAILED DESCRIPTION OF THE EMBODIMENTS
It is well known that the F-P LD shows a multi-mode output and the mode power is proportional to the spontaneous emission coupled to the mode. The output spectral distribution of the F-P LD can be changed by externally injecting a strong light into the F-P LD. Then, a mode that is the nearest from the peak wavelength of the injected light is locked by the injected light and the other modes may be suppressed. Namely, the output wavelength of F-P LD coincides with the peak wavelength injected light. As a result we can obtain a wavelength-selective output from multi-mode laser, F-P LD.
Hereinafter, referring to appended drawings, desirable embodiments of the present invention are described in detail.
FIG. 2 is a schematic diagram of the light source according to the embodiment of the present invention. The light source comprises: an incoherent light source (ILS); a tunable optical filter (TF) connected to said incoherent light source; an optical circulator (CIR) connected to said tunable optical filter; and a F-P LD without optical isolator connected to said optical circulator.
Optionally, the light source according to the embodiment of the present invention further comprises: a polarization controllers (PC) connected between said optical circulator and said F-P LD; and a polarizer (Pol) connected at the output end of said optical circulator.
In the embodiment, the incoherent light source is any one of an optical fiber amplifier generating ASE, an LED, or a SLD.
The operation principles of the light source according to the present embodiment are as follows:
The broadband incoherent light generated from the incoherent light source is sliced by the tunable optical filter to produce a narrow-band incoherent light. The narrow-band incoherent light is injected into the F-P LD through the optical circulator. The optical circulator separates the narrow-band incoherent light and the output of F-P LD. Thus the output of the light source according to the present embodiment comes out through the output end of the optical circulator.
When the F-P LD is biased above the threshold current, the output of the F-P LD is multi-mode. However, it becomes wavelength-selective after injection of the narrow-band incoherent light since a strong light is coupled to a specific mode of the F-P LD. The output wavelength of F-P LD is locked to the injected incoherent light and thus can be tuned by changing the pass-band of the tunable optical filter.
The output power of the F-P LD can be changed by controlling the bias current applied to the F-P LD. Thus, we can modulate the light source directly. When the bias current is lower than the threshold current, the output of the light source is a reflected incoherent light at the interface of the pig-tailing fiber and the air. The output of F-P LD is polarized but reflected incoherent light is unpolarized. Using this characteristics, the extinction ratio of the modulated signal can be improved by further comprising a polarization controller (PC) and a polarizer (Pol).
In the light source according to the present embodiment, an optical circulator (CIR) can be replaced by an optical power splitter.
Using the same principles as that of the embodiments described above, multi-channel WDM light source can be implemented.
FIG. 3 shows schematic diagram of the multi-channel WDM light source in accordance with the embodiment of the present invention.
The multi-channel WDM light source comprises: an incoherent light source (ILS); an optical circulator (CIR) connected to said incoherent light source; a (de)multiplexer ((D)MUX) connected to said optical circulator; and plurality of F-P LDs without optical isolator connected at the output end of the said (de)multiplexer.
If the bandwidth of the incoherent light generated said incoherent light source is larger than the free spectral range (FSR) of said (de)multiplexer, the light source further comprises a band-pass filter (BPF) that is connected between said optical circulator (CIR) and said (de)multiplexer. The band-pass filter restricts the bandwidth of the incoherent light entering the (de)multiplexer within the free spectral range (FSR) of an the (de)multiplexer.
Optionally, the light source further comprises: plurality of polarization controllers (PC) connected between the output ends of the said (de)multiplexer and said F-P LDs; and a polarizer (Pol) connected at the output end of said optical circulator. In the embodiment, the incoherent light source is any one of an optical fiber amplifier generating ASE, an LED, or a SLD.
The operation principles of the multi-channel WDM light source in the present embodiment is as follows: The broadband incoherent light generated from the incoherent light source is transmitted to the (de)multiplexer through the optical circulator. The (de)multiplexer receives and slices the broadband incoherent light. Then, the sliced narrow-band incoherent light with different wavelengths are injected simultaneously into the plurality of F-P LDs.
After injection of incoherent light, the output of each F-P LD becomes wavelength-selective and is locked by the injected narrow-band incoherent light. Namely, the output wavelength of each F-P LD coincides with the peak wavelength of the (de)multiplexer pass-band. The outputs of the F-P LDs are multiplexed by the (de)multiplexer. Then, the multi-channel WDM signals come out through the output end of the optical circulator.
The output power of multi-channel WDM light source can be controlled independently and thus multi-channel WDM light source can be modulated directly. We can increase the extinction ratio of the modulated signal by further comprising a polarizer (Pol) and plurality of polarization controllers (PC).
In the multi-channel WDM light source according to the present embodiment, an optical circulator (CIR) can be replaced by an optical power splitter.
FIG. 4a shows a schematic diagram the optical transmission system for upstream signal transmission in a passive optical network using the multi-channel WDM light source in accordance with the present invention.
The passive optical network of the present embodiment comprises a central office, a remote node connected to the central office with a single optical fiber, and plurality of optical network units connected to the remote node with plurality of optical fibers;
wherein the central office comprises: an incoherent light source (ILS); a demultiplexer (DMUX); an optical circulator that route the output of said incoherent light source to the optical fiber connecting said central office and said remote and the upstream signal transmitted from said remote through said optical fiber to said demultiplexer; and plurality of receivers (RX) connected at the output ends of the said demultiplexer,
the remote node comprises: an (de)multiplexer that receives the broadband incoherent light transmitted from said central offices, slices said incoherent light spectrally to produce plurality of narrow-band incoherent lights and multiplexes the upstream signals from said optical network units, and
the plurality of optical network units comprise a F-P LD that is connected to the output ends of the (de)multiplexer in the remote node with said plurality of optical fibers.
Under this configuration, the upstream signals generated from the optical network units have different wavelengths and multi-channel WDM signal is transmitted from the remote node to the central office.
In the passive optical network, electric power is not supplied to the remote node to save the maintenance cost, and thereby the pass-band of the (de)multiplexer in remote node can drift with the temperature change. Therefore, it is important to control the wavelength of the light sources in the optical network units. In case of the passive optical network using the multi-channel WDM light source according to the present invention, the output wavelength of each F-P LD is automatically aligned to the pass-band of the (de)multiplexer in remote node since the output wavelength of the F-P LD is locked by the injected incoherent light.
In the passive optical network described above, the broadband incoherent light transmitted from the central office to the remote node may be reflected to the central office due to the Rayleigh back-scattering of the optical fiber. The reflected light can degrade the signal quality.
FIG. 4b shows a schematic diagram of the optical transmission system for upstream signal transmission in a passive optical network to reduce the signal degradation described above.
As described in the figure, by installing an optical circulator (CIR) at the remote node and separating the optical fiber that delivers the incoherent light from the optical fiber that deliver the upstream signal, the signal degradation caused by the reflection of the incoherent light can be reduced.
In other words, the passive optical network of the present embodiment comprises a central office, a remote node connected said central office with two optical fibers, and plurality of optical network units connected to said remote node with plurality of optical fibers;
wherein the central office comprises: an incoherent light source (ILS) connected to said remote node with an optical fiber; a demultiplexer (DMUX) connected to said remote with the other optical fiber and plurality of receivers (RX) connected at the output ends of the said demultiplexer,
the remote node comprises: a (de)multiplexer that receives the broadband incoherent light transmitted from the central offices, slices said incoherent light spectrally to produce plurality of narrow-band incoherent lights, and multiplexes the upstream signals from said optical network units; and an optical circulator that route the broad-band incoherent light transmitted from said central office to said (de)multiplexer and the upstream signals from said (de)multiplexer to the central office, and
the plurality of optical network units comprise F-P LDs connected to the output ends of the (de)multiplexer in the remote node with said plurality optical fibers.
Under this configuration, the upstream signals generated from the optical network units have different wavelengths and multi-channel WDM signal is transmitted from the remote node to the central office.
In optical transmission system for upstream signal transmission in a passive optical network described in FIG. 4a and FIG. 4b, an optical circulator (CIR) can be replaced by an optical power splitter.
FIG. 5 shows the experimental set-up to demonstrate the feasibility of the light source in accordance with the present invention.
The ASE source was two-stage erbium-doped fiber amplifier (EDFA) pumped counter-directionally with laser diode at 1480 nm. The pump power for the first and the second stage of EDFA were 50 mW and 100 mW, respectively. A band pass filter (BPF) with a bandwidth of 9 nm was used at the output end of the EDFA to limit the spectral width of the ASE within one free spectral range (FSR) of the waveguide grating router (WGR). An optical amplifier (AMP1) and an optical variable attenuator (Att.1) were used to control the ASE power injected into the F-P LD. An optical circulator with insertion loss of 0.7 dB separated the injected broadband ASE and the output of the F-P LD. The broadband ASE was sliced spectrally by an WGR with a bandwidth of 0.24 nm and injected into the F-P LD. A conventional F-P LD without an optical isolator was locked by the externally injected narrow-band ASE. The threshold current of the F-P LD was 20 mA. The coupling efficiency of the F-P LD, the rate of power transferred from laser to pig-tailing fiber or vice versa, was approximately 8%. The F-P LD was modulated directly by pseudorandom nonreturn-to-zero data with a length of 27−1 at 155 Mb/s and its output was transmitted through conventional single mode fiber (SMF). The transmitted data was amplified by an optical amplifier (AMP2), demultiplexed by another WGR with a bandwidth of 0.32 nm, and received by a PIN photo-detector based receiver to measure the bit error rate (BER) characteristics. The receiver input power was controlled by an optical variable attenuator (Att.2) and measured by an optical power meter (PM). A polarization controller (PC) and a polarizing fiber (PZF) with about 47 dB of polarization extinction ratio are used to improve the extinction ratio of the modulated optical signal.
FIG. 6 shows (a) the output spectrum of the F-P LD without ASE injection and (b) the spectrum of the narrow-band ASE to be injected into the F-P LD. The bias current was 30 mA and the output power of the F-P LD measured at the output end of the optical circulator was about −10 dBm. The side mode suppression ratio (SMSR) was less than 6 dB. The peak wavelength of narrow-band ASE was about 1551.72 nm.
FIG. 7 shows the measured output spectra of the F-P LD after injection of a narrow-band ASE when the injected ASE power were (a) −2 dBm and (b) 2 dBm, respectively. After ASE injection, the F-P LD was wavelength-locked by the injected ASE. The measured side mode suppression ratio were 25 dB and 27.3 dB for the injection ASE power of −2 dBm and 2 dBm, respectively.
FIG. 8 shows the measured side mode suppression ratio (SMSR) of the light source in accordance with the present invention. The side mode suppression ratios increases as the injected ASE power increases. However, it decreases as the bias current increases.
To measure the modulation characteristics of the light source in accordance with the present invention, we measured optical spectra for different bias currents at the fixed injection ASE power of 2 dBm. FIG. 9 shows the results when the bias current were 30 mA (dotted line) and 0 mA (solid line), respectively. The measured peak power difference between two bias states, here called as extinction ratio, was about 5.8 dB.
FIG. 10 shows the measured the extinction ratio of the light source in accordance with the present invention. The extinction ratio decreases as the injection ASE power increases while it increases the as the bias current increases.
We also measured optical spectra by inserting a polarization controller and a polarizer (in the present experiment, a polarizing fiber: PZF) under the same measurement conditions with the FIG. 9. FIG. 11 shows the results. The extinction ratio increases about 2.5 dB from 5.8 dB to 8.3 dB. This means that the output of the light source according to the present invention is polarized.
FIG. 12 shows the measured bit error rate curves. The F-P LD was modulated directly at 155 Mb/s. The amplitudes of dc bias and modulation current were both 20 mA. Before we use the light source according to the present invention, we measured BER characteristics of the directly modulated F-P LD itself, i.e., without ASE injection. The measured power penalty at the BER of 10−9 was about 2 dB after transmission over 20 km of SMF as shown in FIG. 12 (a).
The BER characteristics were improved dramatically when we inject a narrow-band ASE into the F-P LD. The power and the peak wavelength of the injected ASE were 1 dBm and 1551.72 nm, respectively. We achieved error free transmission over 120 km of SMF with negligible power penalty as shown in FIG. 12 (b). We also measured BER characteristics by changing the peak wavelength of the injected narrow-band ASE and observed very similar results. As an example, we show the measured BER curves in FIG. 12 (c) when the peak wavelength of the injected narrow-band ASE was 1550.92 nm. This result implies that the output wavelength of the light according to the present invention can be tuned by changing the wavelength of the injected ASE.
Since those having ordinary knowledge and skill in the art of the present invention will recognize additional modifications and applications within the scope thereof, the present invention is not limited to the embodiments and drawings described above.

## Claims

1. A multi-channel WDM light source comprising:
an incoherent light source that generates a broadband incoherent light;
an optical circulator coupled to the incoherent light source;
a demultiplexer coupled to the optical circulator, wherein the optical circulator is configured to route the broadband incoherent light to the demultiplexer and to separate an output of the demultiplexer from the broadband incoherent light, wherein the demultiplexer is configured to receive the broadband incoherent light, to slice spectrally the broadband incoherent light to produce a plurality of narrow-band incoherent lights; and
a plurality of light sources capable of lasing that are connected at output ends of the demultiplexer, wherein the demultiplexer is further configured to multiplex outputs of the plurality of light sources capable of lasing, wherein at least one of the plurality of light sources capable of lasing is configured to emit a wavelength-selective output locked by one of the narrow-band incoherent lights, wherein the at least one of the plurality of light sources capable of lasing is configured to be modulated directly, wherein the at least one of the plurality of light sources capable of lasing is configured to have a side mode suppression ratio of the wavelength-selective output increased by increasing a power of the one of the narrow-band incoherent lights.

2. The multi-channel WDM light source of claim 1, wherein the plurality of light sources comprise Fabry-Perot laser diodes.

3. The multi-channel WDM light source of claim 1, wherein the broadband incoherent light has a bandwidth within a free spectral range (FSR) of the demultiplexer.

4. The multi-channel WDM light source of claim 1, further comprising of polarization controllers between the output ends of the demultiplexer and the plurality of light sources.

5. The multi-channel WDM light source of claim 1, further comprising a polarizer coupled to an output of the optical circulator.

6. The multi-channel WDM light source of claim 1, further comprising an optical power splitter that replaces the optical circulator that is coupled to the incoherent light source and to the demultiplexer.

7. The multi-channel WDM light source of claim 1, further comprising a band pass filter between the demultiplexer and the optical circulator.

8. The multi-channel WDM light source of claim 1, wherein a bias current is applied to the at least one of the light sources capable of lasing, and wherein the at least one of the light sources capable of lasing is configured to have the side mode suppression ratio of the wavelength locked output decreased by increasing the bias current.