Abstract:
An external modulation optical communication system suppressing the intensity noise of a semiconductor laser source by making substantially different optical path lengths for the optical paths in the interferometric modulator. Noise reduction is dependent on the correlation between intensity noise and frequency noise of the semiconductor laser. The interferometer path delay difference forms an optical filter that transforms frequency fluctuations into intensity fluctuations that are out of phase with the laser intensity fluctuations. The intensity modulation induced by frequency noise can be out of phase with the optical source intensity fluctuations, leading to total intensity noise at the output of the optical link which is lower than the laser intensity noise. Because the optical interferometer is already present for optical modulation, the optical filter realized by the unequal path delays of the interferometer does not add optical loss. A high correlation between intensity and frequency noise of the optical source is required for substantial intensity noise reduction.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under Contract Number F19628-95-C-0002 awarded by the U.S. Air Force. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of optical signal transmission and in particular to the reduction of intensity noise in such systems. 
     BACKGROUND OF THE INVENTION 
     Semiconductor lasers generally are an inexpensive source of coherent optical power. Semiconductor lasers can be used in optical communication systems to transmit electrical signals over optical fiber using direct modulation or external modulation. In direct modulation, an electrical input signal modulates the bias current of the laser. In external modulation, a continuous-wave (CW) laser is used and the electrical input signal modulates the transmission of light through an optical modulator. Typically the modulator is an electro-optic crystal placed in an interferometer to convert phase modulation to intensity modulation. 
     The performance of optical systems using semiconductor lasers is degraded by the high intensity noise produced by the semiconductor laser. This intensity noise manifests itself as electrical noise at the output of the optical system. For high dynamic range applications such as communicating with remotely-located radar antennas, analog optical systems are required to have both low noise and low distortion. Typically, in optical communication systems an expensive Nd:YAG laser is used as the laser source due to its low noise. A semiconductor diode laser is potentially the lowest-cost laser source useful for external modulation, however, it requires noise suppression to be an effective optical source for communication systems using external modulation. The intensity noise of a semiconductor laser can be reduced by passing the light through an optical filter which has the appropriate variation in transmission as a function of wavelength. 
     In a semiconductor laser or semiconductor amplifier, fluctuations of the carrier density level that determines the material gain is the source of intensity noise. Changes in the carrier density change both the gain and refractive index of the gain medium. The ratio between the real part of the refractive index to the imaginary part of the refractive index (which is proportional to the gain) is defined as the linewidth enhancement factor a. When the laser is modulated, a large linewidth enhancement factor a produces a large frequency modulation (FM) signal corresponding to the intensity modulation. Because of this correlation between gain and phase changes in the gain medium, carrier fluctuations also produce optical FM noise that is correlated with the intensity noise of the laser. Thus, if the optical signal is passed through an optical filter having the appropriate transfer function slope, the FM noise of the optical carrier generates correlated intensity variations at the output of the optical filter. These correlated intensity variations can be out of phase with the original intensity noise, leading to substantially lower total intensity noise. 
     An optical filter noise reduction technique has generally not been applied to semiconductor laser optical communication systems because the method as previously disclosed has not been suitable in such systems using direct modulation or external modulation links. This is because if the optical filter noise reduction technique were applied to a direct modulation link, the signal to noise ratio at the link output would be reduced because the modulation signal would be reduced by at least as much as the intensity noise. If this optical filter noise reduction technique were applied to an external modulation communication system, the optical insertion loss of the noise reduction filter can lead to unacceptable degradation of link performance. Therefore, what is needed is an external modulation configuration with an optical filter that has the appropriate transmission slope for noise reduction, but does not introduce optical insertion loss. 
     SUMMARY OF THE INVENTION 
     The invention relates to a method and apparatus for modulating the intensity and improving the noise figure of an optical source in an optical communication system. An optical filter is used to reduce phase correlated intensity noise in the system without adding optical insertion loss. The filter is included in an interferometer having unbalanced (i.e., unequal) path lengths and an internal phase modulator. The method and apparatus are suitable for laser sources in which the refractive index of the laser medium varies with a change in gain. For example, links using semiconductor diode laser sources can benefit from this method. 
     The apparatus includes an optical source and an interferometer. The interferometer includes an input port, an output port, a first optical path and a second optical path. The first and second optical paths are each in optical communication with the input port and the output port. The length of the second optical path differs from the length of the first optical path by a predetermined optical path length difference. The apparatus also includes a modulator having a modulator electrical input. The modulator is located in the second optical path of the interferometer. The modulator generates an optical phase delay in response to a signal received at the modulator electrical input. The interferometer provides an optical signal having reduced phase correlated intensity noise in response to the predetermined optical path length difference. 
     In one embodiment the optical source is a semiconductor laser diode. In another embodiment the apparatus includes a detector in optical communication with the interferometer output port. In another embodiment the predetermined optical path length difference is adjustable. 
     In one embodiment the apparatus includes a reflective element in which receives radiation transmitted by the first and second optical paths through the output port. The received radiation is reflected back into the output port and transmitted back through the first and second optical paths. 
     The method includes the steps of providing a source of optical radiation having intensity noise, splitting the optical radiation from the source into a first optical signal and a second optical signal, and delaying the optical phase of the second optical signal relative to the first optical signal by a predetermined phase delay. The method also includes the steps of modulating the phase of the first or second optical signal in response to an electrical signal and combining the first and second optical signals to generate an output optical signal having a reduced intensity noise. In one embodiment the predetermined phase delay produces a periodic variation in interferometer output intensity as a function of wavelength that is an integer multiple of the mode spacing of the optical source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a highly schematic diagram of an embodiment of an external modulation communication system constructed according to the present invention. 
     FIGS. 2A and 2B are highly schematic diagrams of an embodiment of an external modulation communication system using an unequal path length Michelson interferometer filter and an unequal path length Mach-Zehnder interferometer filter, respectively, for optical noise reduction. 
     FIGS. 3A and 3B are highly schematic diagrams of an embodiment of an external modulation communication system using an unbalanced interferometer configuration utilizing a linearized series Mach-Zehnder modulator and a linearized reflective half-coupler modulator, respectively, according to the present invention. 
     FIGS. 4A to  4 C are diagrams of the transmission of the noise reduction optical filter and the spectrum of the optical source for an embodiment of the invention having an optical source having a single optical mode, a Fabry-Perot laser source having an optical mode spacing equal to the optical filter periodicity, and a Fabry-Perot laser source where the optical filter periodicity is equal to an integer multiple of the optical mode spacing of the Fabry-Perot laser, respectively. 
     FIG. 5 is a highly schematic diagram of an embodiment of the present invention. 
     FIG. 6 is a graph of the measured modulation enhancement of the optical filter noise reduction circuit as a function of the interferometer path length difference for the embodiment of the invention shown in FIG.  5 . 
     FIG. 7 is a graph of the measured noise enhancement of the optical filter noise reduction circuit as a function of the interferometer path length difference for the embodiment of the invention shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an unbalanced interferometer external modulation communication system  100  includes a laser source  10 , a Mach-Zehnder modulator  16  having paths  20  and  22  of unequal optical path lengths due to path length difference  24 , a phase modulator  26  and a detector  36 . The phase modulator  26  provides a differential phase shift between the optical signals in the two paths  20  and  22  in response to an electrical signal V RF . The detector  36  converts the modulated optical output signal from the interferometer  16  to an electrical output signal V OUT  at the detector output  37 . The laser  10  provides light with correlated intensity and frequency noise. The laser  10  can be a semiconductor laser or other type of laser and can include an optical amplifier such as a semiconductor optical amplifier. Typically, the laser source  10  includes an optical isolator (not shown) to reduce the power of reflections from components in the communication system  100  from returning to the laser source  10  and degrading its performance. 
     In one embodiment the Mach-Zehnder modulator  16  is formed from two waveguides in an electro-optic substrate. Light from the laser source  10  is transmitted through an optical fiber  12  to an optical power splitter  18 . The light is directly coupled into the optical power splitter  18  by close proximity or physical contact between the optical fiber  12  and the electro-optic substrate. In another embodiment, the optical fiber  12  is coupled to the optical power splitter  18  with an external lens or a lens attached or formed on the end of optical fiber  12 . The optical power splitter  18  directs approximately half the laser source output power into each of the two optical paths  20  and  22 . One path  20  has additional optical delay due to additional waveguide length  24 . This additional path length results in a wavelength dependence in the transfer function used as the optical filter for noise reduction. 
     In one embodiment, the substrate is an electro-optic material such as lithium niobate (LiNbO 3 ). The electrical signal V RF  is applied to one or more electrodes on the substrate. The electrodes produce an electric field in the electro-optic substrate proportional to the electrical signal V RF  resulting in a relative phase delay between the two unequal optical paths  20  and  22 . The two optical paths  20  and  22  are interferometrically combined in an optical power combiner  28 . The optical power splitter  18  and combiner  28  can be formed from optical fiber couplers, other dielectric waveguide couplers, Y-branch splitters, multi-mode interference couplers, or partially reflecting mirrors. The optical power combiner  28  is coupled into an optical fiber  34 . The modulated optical output from optical fiber  34  is converted to an electrical signal by detector  36 . The detector can be any device used to convert the intensity modulation of the optical signal into an electrical signal, including for example semiconductor photodiodes and phototransistors. 
     The optical bandwidth of the Mach Zehnder noise reduction filter  16  is set by the optical delay difference (Δt filt ) between the two optical paths  22  and  24  in the interferometer  16 . A short delay difference Δt filt  results in a very broadband optical filter bandwidth and, conversely, a large delay difference Δt filt  results in a narrow filter bandwidth. The amount of FM to intensity modulation conversion depends on the local slope of the noise-reduction optical filter  16  or, equivalently, the change in optical transmission of the noise-reduction optical filter  16  as a function of the change in wavelength. This FM to intensity conversion efficiency is determined mainly by the optical path delay difference Δt filt  which can be many optical wavelengths, however, the filter slope is also sensitive to small changes in Δt filt  on the order of an optical wavelength. If the change in Δt filt  is on the order of an optical wavelength then the relative optical phase between the two paths  22  and  24  is sufficiently changed so that the average optical transmission of the filter  16  is generally changed. 
     A modulated output signal from a conventional Mach-Zehnder modulator  50  transmitted through an optical fiber  34  to an external optical filter  60  used by the prior art to reduce intensity noise from the laser source  10  is shown in FIG.  2 A. The optical noise reduction filter  60  includes a lens element  62  and a Michelson interferometer  40  constructed of free space optical elements  64 ,  66  and  68 . The lens  62  converts the output of the optical fiber  34  to a collimated beam that is incident on a freespace optical power splitter  64 . The optical power splitter  64  splits the free space optical beam into two approximately equal power optical beams  42  and  44 . Optical reflectors  66  and  68  reflect the optical beams back to the optical power splitter  64  which also acts as an optical power combiner by interferometrically summing the two reflected beams  42  and  44  into a single noise-filtered optical beam  46 . The photodetector  36  converts the modulated, noise-filtered optical beam to a modulated electrical signal V OUT  at the detector output  37 . 
     The Michelson interferometer  40  of FIG. 2A can be replaced with a Mach-Zehnder noise reduction filter  48  as shown in FIG. 2B. A second Mach-Zehnder interferometer  50  similar to the Mach-Zehnder interferometer  50  of FIG. 2A is used to modulate light from the source optical fiber  12  in response to an electrical input signal V RF . Although the output of the Mach-Zehnder modulator  50  is shown coupled by an optical fiber  52  to the Mach-Zehnder optical filter  48  on a separate substrate, the Mach-Zehnder modulator  50  can be monolithically integrated on the same substrate as the Mach-Zehnder filter  48 . An electrical control input (V filt ) to the phase modulator  26  sets the relative transmission of the Mach-Zehnder optical filter  48  at the optical wavelength of the laser source  10 . The optical output signal from the optical filter  48  is transmitted through optical fiber  34  and converted to an electrical output by the photodetector  36 . 
     Referring back to FIG. 1, a noise-reduction optical filter according to the present invention combines the Mach-Zehnder optical modulator  50  used for intensity modulation and the Mach-Zehnder optical filter  48  for optical filtering in FIG. 2B into a single interferometer  16 . By combining the filter  48  and the modulator  50 , the total optical loss introduced in the system can be reduced to that of the intensity modulator  50 . This gives a system noise performance which is equivalent to using a lower intensity noise optical source without reducing the optical power received at the photodetector  36 . A disadvantage of previous optical filter noise reduction implementations is that the optical filter  16  must have some slope in its optical transfer function, which means that there is necessarily some optical filter loss. For external modulation links, the optical filter loss results in a higher noise figure, lower gain and dynamic range, and additional components. 
     The transfer function of an interferometric modulator  50  is a sinusoidal function of its input electrical signal V RF . Consequently, the nonlinear transfer function of the optical modulators  16 ,  48  and  50  shown in FIG.  1  and FIG. 2 generate undesirable distortion products. In order to reduce distortion, a number of linearization techniques can be implemented. One common technique is to introduce predistortion in the electrical signal V RF  used to drive the intensity modulators. Similarly, postdistortion can be added to the link output signal V OUT  by an electrical circuit which adds distortion to the electrical signal V OUT  to reduce or cancel the distortion of the optical link. 
     Optical linearization is another technique for decreasing the distortion and increasing the dynamic range of external modulation links. Many optical linearization configurations use two optical components with nonlinearities of opposite signs that are summed together to give improved link linearity. One configuration for this technique uses a series of Mach-Zehnder modulators  70  and  84  with unequal path lengths as shown in FIG. 3A in place of the single Mach-Zehnder modulator  16  with unequal path lengths shown in FIG.  1 . Light from the optical source  10  passes through two Mach-Zehnder modulators  70  and  84  in series. The electrical input signal V RF  is split by an RF power splitter  96  and applied to both phase modulators  80  and  94 . The third-order distortion component is completely cancelled with the appropriate splitting of the RF electrical input power and setting of the bias points (i.e., quiescent operating points on the voltage to intensity transfer function) for both phase modulators  80  and  94 . Using unequal path delays  78  and  92  in one or both modulators results in a wavelength dependent optical filter transfer function useful for suppressing source intensity noise. 
     Referring to FIG. 3B, another configuration for optical linearization is based upon another unbalanced optical modulator  120  that includes an optical reflective element  134 . The output of the laser source  10  is coupled by an optical fiber  12  to an optical directional coupler  122  at the input of the half-coupler modulator  120 . The optical directional coupler  122  is a bidirectional two-port device that couples the laser output to paths  124  and  126 . A differential delay  128  is added to one optical path  124 . A phase shifter  130  applies a phase shift difference between paths  124  and  126  proportional to the electrical input signal V RF . The paths  124  and  126  are coupled together by a second bidirectional two-port optical coupler  132 . Optical mirror  134  reflects the light from coupler  132  back through the coupler  132 . The reversed light in paths  124  and  126  are formed of linear combinations of forward travelling light in paths  124  and  126 , with additional phase shift due to phase shifter  130  and additional differential delay due to optical delay  128  in the reverse direction. Thus the amplitude of light travelling in a reverse direction in paths  12  and  34  are linear combinations of the amplitude of reverse travelling light in paths  124  and  126 . Linearization properties of the half-coupler modulator  120  are dependent on the amount of coupling achieved by optical directional couplers  122  and  132 , and the bias point set by the phase shifter  130 . 
     FIG. 4A shows the transfer function  82  of the optical signal generated with a single frequency optical source (e.g., a distributed feedback (DFB) laser) and a Mach-Zehnder modulator  16  or  48 . A Fabry-Perot laser is easier and considerable less expensive to fabricate than the DFB laser, however, the Fabry-Perot laser produces multiple optical modes  98 . Generally there will be a different optical filter slope  94  at the wavelength λ of each optical mode  98  of the laser  10 . The transfer function  82  of an unequal path delay interferometer optical filter repeats periodically with a fixed optical frequency spacing T corresponding to a wavelength spacing of λ T . The optical modes  98  of the Fabry-Perot laser are approximately equally spaced by Δλ in wavelength which corresponds the mode frequency separation. Thus a Fabry-Perot laser can be used as a source if the wavelength spacing λ T  of the optical filter is equal to the wavelength spacing Δλ of the laser optical modes  98  as shown in FIG.  4 B. The wavelength spacing λ T  of the optical filter can also be set to a multiple of the Fabry-Perot mode spacing Δλ as shown in FIG.  4 C. 
     An experimental configuration for an embodiment of a noise-reduction optical filter using an unbalanced interferometer  156  is shown in FIG.  5 . The laser source  10  is a 1.54 μm DFB laser (ORTEL, Alhambra, Calif., model no. 1740A). The Mach-Zehnder unbalanced interferometer  156  shown uses a fiber-optic splitter  140  and combiner  154 . The input optical power splitter  140  has an unequal 30:70 optical power splitting ratio to compensate for the coupling loss of a fiber-coupled phase modulator  152  (Uniphase Telecom Products, Bloomfield, Conn., phase modulator model no. PM-150-005-00-1-1-B) in one path  148  of the interferometer. The fiber-optic combiner  154  has a 50:50 coupling ratio. An adjustable time delay unit  150  (Princeton Optics, Princeton, N.J., fiber optic delay line model no. 50-15-25) was used in one arm of the interferometer to vary the slope of the filter function. The half-wave voltage (i.e., the voltage required to delay one path of the interferometer relative to the other path by one-half the wavelength) of the phase modulator  152  is 1.3 V at 103 MHz. 
     In the absence of uncorrelated FM noise from the laser source  10 , the total intensity noise in the optical signal in optical path  34  is the combination of the AM intensity noise of the laser source  10  and the intensity noise generated by the interferometer  156  from the correlated FM noise of the laser source  10 . The total intensity noise is minimized by using an interferometer path length difference which is approximately equal to the path length difference required to minimize the modulation signal applied to the laser source  10 . If the laser source  10  also has uncorrelated FM noise, the optical signal in optical path  34  includes a third noise component which is generated by the interferometer  156  from the uncorrelated FM noise of the laser source  10 . 
     The intensity noise generated in the interferometer  156  by the FM noise in the laser source  10  is proportional to the optical path length difference selected with the time delay unit  150 . The optimum path length difference for minimizing the total intensity noise is less for a laser source  10  having both correlated and uncorrelated FM noise components than for a laser source  10  having only a correlated FM noise component. The minimum total intensity noise achievable at the optimum optical path length difference is greater for a laser source  10  having both correlated and uncorrelated FM noise components than it is for a laser source  10  having only a correlated FM noise component. 
     Suppression of optical intensity noise resulting from a noise current injected into the laser source  10  was determined by intentionally modulating the laser source  10  and measuring the resulting modulation suppression. The laser source  10  was modulated at 103 MHz and the RF power was measured at the output  37  of the photodetector  36 . The relative link gain is shown as a function of the path length difference in FIG. 6 for a laser bias of 27 mA and the interferometer  156  set at quadrature. The resulting detector photocurrent was 0.5 mA and the maximum modulation suppression was 5.7 dB. The upper curve  160  represents the modulation enhancement measured when the AM modulation resulting from the FM to AM conversion of the FM modulation is in-phase with the optical signal from the AM modulated laser source  10 . The lower curve  162  represents the modulation enhancement measured when the AM modulation resulting from the FM to AM conversion is out-of-phase with the optical signal from the AM modulated laser  10 . A maximum suppression of approximately 6 dB is achieved. As the differential path length increases beyond the 6 dB suppression length (not shown), the modulation enhancement for the out-of-phase conversion  162  increases because the total modulation is dominated by the converted FM modulation which is proportional to the differential path length. 
     The excess noise at the photodetector  36  measured using a noise figure meter is shown as a function of the difference in interferometer path lengths in FIG.  7 . Excess noise is defined as the difference between the total output noise and the thermal noise. For a laser bias of 27 mA and detector photocurrent of 0.5 mA, the maximum noise suppression was 1.3 dB. For the unbalanced modulator configuration, there is no extra loss (i.e., insertion loss) from the insertion of an optical filter element therefore the link gain does not change when the optical path lengths are different. Consequently, the improvement in link noise figure is 1.3 dB, the same as the reduction in excess noise due to the optical filtering performed by the unbalanced interferometer  156 . 
     For perfect correlation between intensity noise and FM noise, the amount of relative intensity noise reduction in FIG. 7 is equal to the amount of suppression of modulation from the injected modulation signal used in the measurements of FIG.  6 . The path length difference for optimum noise suppression, however, turns out to be much shorter than that for optimum modulation suppression because the uncorrelated FM noise of the laser source  10  causes the total intensity noise to increase for larger path length differences. This correlation between intensity noise and FM noise typically degrades at higher optical power. 
     While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail made be made therein without departing from the spirit and scope of the invention as defined by the appended claims.