All-optical analog FM optical receiver

[0003] The invention relates generally to an apparatus and method for an optical communications system, and in particular, to an analog frequency modulated (FM) optical receiver.

[0004] Communication systems based on optical technologies are becoming more common due to advantages over conventional wire-based communication systems. Although digital optical links can provide high data bandwidths, in some implementations analog optical links are preferred, for example where providing digital processing capability at the transmitter is impractical.

[0005] An analog optical link can exhibit an unacceptable noise figure (NF) and an unacceptable spurious-signal-free dynamic range (SFDR). For example, conventional Mach-Zehnder modulated optical links are subject in general to second order harmonics and third order difference intermodulation products. Thus a need exists for a simple and inexpensive high performance analog optical link having a low noise figure and a high SFDR.

[0006] The present invention relates to an apparatus and method for all-optical, frequency modulated (FM) communication. The method and apparatus make use of a balanced receiver configuration having a set of optical filters. An optical beam splitter provides an optical signal to two distinct optical filters. One optical filter removes the upper sideband and the other optical filter removes the lower sideband. The filtered signals are detected and their photocurrents subtracted before the resulting electrical signal is provided to a differentiator for demodulation. Laser relative intensity noise (RIN) and second order harmonics are thereby eliminated. In addition, third order distortion is eliminated when no intensity modulation (IM) is present, or greatly reduced when IM is present, As a result, this analog optical link has a low noise figure and a high spurious-signal-free dynamic range.

[0007] The present invention features a transmitter for an optical communication system which includes a FM optical signal source and an optical filter in communication with the FM source. The optical filter produces a single sideband optical signal. In one embodiment the transmitter also includes a beamsplitter and a second optical filter. The beamsplitter is in optical communication with the FM optical signal source and has a first and second beam output. Each optical filter is in communication with a respective beam output and removes one sideband of the FM optical signal.

[0008] The invention also features a receiver for an optical communication system which includes an optical filter, a photodetector in optical communication with the filter, and a differentiator in electrical communication with the photodetector. The optical filter removes one sideband of a FM optical signal to produce a single sideband FM optical signal and the differentiator produces a demodulated electrical signal in response to the single sideband FM optical signal. In one embodiment the receiver also includes a beamsplitter and a second optical filter. The beamsplitter is in optical communication with the FM optical signal source and has a first and second beam output. Each optical filter is in communication with a respective beam output and produces a respective single sideband FM optical signal in response to the FM optical signal.

[0009] The invention also features a communication system which includes a FM optical signal source, an optical filter in optical communication with the FM optical signal source, a photodetector in optical communication with the filter and a differentiator in electrical communication with the photodetector. The differentiator produces a demodulated FM electrical signal in response to the FM optical signal. In one embodiment the system also includes a beam splitter, a second filter and a second photodetector in electrical communication with the differentiator. The differentiator produces a demodulated electrical signal in response to the detected electrical signals from the photodetectors.

[0010] The foregoing and other objects, features and advantages of the invention will become apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being placed on illustrating the principles of the present invention.

[0011] FIG. 1 is a functional block diagram illustrating one embodiment of an all-optical FM analog link;

[0012] FIG. 2 is a functional block diagram illustrating another embodiment of an all-optical FM analog link;

[0013] FIGS. 3a-3dare diagrams of the filtered optical spectrums according to the invention;

[0014] FIG. 4 is a functional block diagram of an embodiment of an all-optical FM analog link using Fabry-Perot filters;

[0015] FIGS. 5aand 5bare diagrams of the filtered optical spectrums of the embodiment of FIG. 4.

[0016] Referring to FIG. 1, an all-optical FM link 10 includes a laser source 12 which can be directly modulated, an optical fiber 14 and a receiver 16. Preferably, the laser 12 is a distributed feedback (DFB) laser (e.g. KBK Inc., New York, N.Y., DFB laser model no. KLED1563BTB which has a FM response efficiency of 50-500 MHz/mA, depending on the modulation frequency, and a bandwidth up to 3 GHz) which is directly modulated by a RF signal (RF_in). The resulting modulated optical signal includes both intensity modulation (IM) and frequency modulation (FM). The receiver includes an optical filter 20, a photodetector 22 and a differentiator 24. The optical filter 20 suppresses one sideband of the optical signal and passes both the carrier and the other sideband. As an example, a fiber grating filter can be used in a fiber-based receiver. Generally, any transmissive optical filter with the proper spectral characteristics can be used. Without the optical filter 20 the FM modulation is canceled and only the IM is detected. Demodulation of the electrical signal generated by the photodetector 22 is accomplished with the differentiator 24 and results in an output RF signal (RF_out) at the receiver output terminal 18.

[0017] Other lasers that can be directly modulated and have a single longitudinal mode can be used in place of the conventional DFB laser 12. Semiconductor lasers which satisfy these requirements include, but are not limited to, distributed Bragg reflector (DBR) lasers, external cavity lasers and multiple quantum well (MQW) lasers. In addition, some solid state lasers (e.g., Nd:YAG lasers) can be directly modulated, although the modulation efficiency and modulation bandwidth are typically low.

[0018] Unlike a coherent analog FM link, the all-optical FM analog link 10 does not require an additional laser source or strict optical frequency tracking, although the optical frequency must be stable enough to allow proper optical filtering. Laser relative intensity noise (RIN), however, is still present in the demodulated signal (RF_out)

[0019] Referring to FIG. 2, an all-optical FM analog link 10′ having a balanced receiver configuration includes a DFB laser 12, an optical fiber 14 and a dual receiver 16′. The dual receiver 16′ includes a beamsplitter 26, an upper optical filter 20′, a lower optical filter 20′, two photodetectors 22′ and 22″, and a differentiator 24. Modulated light from the optical fiber 14 is split into an upper optical path 28 and a lower optical path 30. The optical paths 28, 30 can include, but are not limited to, optical fiber or a free space path defined by bulk optical components. Light in the upper path 28 passes through the upper filter 20′ where the lower sideband is removed from the modulated optical signal. The upper detector 22′ receives the filtered single sideband optical signal and generates a corresponding modulated electrical signal at the detector output 23. Similarly, light in the lower path 30 passes through the lower optical filter 20′ where the upper sideband is removed from the modulated optical signal. The second detector 22′ receives this second filtered single sideband optical signal and generates a corresponding modulated electrical signal at the detector output 25. A resulting sum modulated signal at node 32 is differentiated by the differentiator 24 to generate a demodulated electrical signal (RF_out) at the receiver output 18.

[0020] FIG. 3aillustrates how the upper optical filter 20′ removes the lower sideband 36 and transmits the upper sideband 38 and carrier 40 which lie within the passband 42 of the filter 20′. Similarly, FIG. 3billustrates how the lower optical filter 20″ removes the upper sideband 38 and transmits the lower sideband 36 and carrier 40 which lie within the passband 44 of the filter 20″.

[0021] The contribution to the photocurrents generated by the detectors 22′, 22″ due to RIN are equal because the upper RIN sideband is the same as the lower RIN sideband. Thus one advantage of the balanced receiver configuration 10′ is the elimination of RIN. Shot noise and thermal noise in the two paths are not correlated, however, so subtraction of the photocurrents from the detectors 22′, 22″ does not eliminate these noise components from the demodulated electrical signal RF_out. Instead, the shot noise and thermal noise present in each optical path 28, 30 are incoherently summed. In addition, second order harmonics and third order distortion are eliminated when no IM is present. Even if IM exists, both second order harmonics and third order distortion are substantially reduced. As a result the balanced receiver configuration 10′ has a low noise figure and a high spurious-signal-free dynamic range.

[0022] The demodulated RF signal current is given by:

[0023] i_sig=KP_SK_FMx_h(t)  (1)

[0024] where K is the efficiency of the differentiator 24, is the photodetector responsivity, P_Sis the optical power, K_FMis the frequency modulation index and x_h(t) is the normalized RF input signal. The gain of the all-optical FM analog link 10′ is given by: $\begin{array}{cc}G={\left(\Re P_S\right)}^2\left(2\pi {\delta }_{\mathrm{FM}}\right)\frac{R_{\mathrm{out}}}{R_{\mathrm{in}}}& \left(2\right)\end{array}$

[0025] where (δ_FMis the FM efficiency (MHz/mA) of the laser 12 and R_outand R_inare the output and input resistance, respectively.

[0026] Using the ideal filter model illustrated in FIGS. 3A and 3B, the noise figure is approximated by: $\begin{array}{cc}NF=\frac{\eta B{R}_{\mathrm{in}}}{{\left(\Re P_S\right)}^2{\left(\frac{{\delta }_{\mathrm{FM}}}{f_c}\right)}^2}& \left(3\right)\end{array}$

[0027] where η is the receiver noise spectral density, B is the signal bandwidth and f_cis the center frequency of the RF signal (RF_in).

[0028] The spurious-signal-free dynamic range can be calculated assuming a more realistic model of the upper optical filter 20′ and lower optical filter 20″, respectively, as shown in FIGS. 3C and 3D. In particular, the passbands 42′,44′ result in suppressed sidebands 36′,38′ with non-zero power contributions S_up. In addition, the RF input signal (RF_in) is assumed to be a two-tone signal where the tones are at closely spaced RF frequencies. The spurious-signal-free dynamic range is given by: $\begin{array}{cc}\mathrm{SFDR}=4{\left(\frac{{\left(\Re P_S\right)}^2}{\eta B}\right)}^{\frac{2}{3}}\frac{{\left(1-S_{\mathrm{up}}\right)}^2}{{\left(8C_{\mathrm{IM}}^2\left(1-S_{\mathrm{up}}^2\right)+S_{\mathrm{up}}\left(1-S_{\mathrm{up}}\right)\right)}^{\frac{2}{3}}}& \left(4\right)\end{array}$

[0029] where $C_{\mathrm{IM}}=\frac{2\pi f_{c}m}{K_{FM}}$

[0030] and m is the intensity modulation index. While this spurious-signal-free dynamic range depends on the relative values of the FM and AM modulation efficiencies, it will typically be substantially better than conventional links utilizing Mach-Zehnder modulators.

[0031] Referring to FIG. 4, another embodiment of a balanced receiver configuration 16″ includes an upper optical circulator 50 and lower optical circulator 52 (e.g., Kaifa Technology, Sunnyvale, Calif., optical circulator model no. CIR5-M which has a minimum isolation between ports of 40 dB). Modulated light from the laser 12 transmitted through the optical fiber 14 is split into the upper and lower optical paths 28 and 30, respectively. Light in the upper path 28 passes through the upper optical circulator 50 and is reflected from the upper optical filter 20′ except for the lower sideband which is transmitted through the filter 20′ and lost. The reflected optical signal having a single sideband enters the upper optical circulator 50 where it is directed into an upper detector optical path 54 and onto the upper detector 22′. Similarly, light in the lower path 30 passes through the lower optical circulator 52 and is reflected from the lower optical filter 20″ except for the upper sideband which is transmitted through the filter 20″ and lost. This second reflected optical signal also has a single (opposite) sideband and enters the lower optical circulator 52 where it is directed into a lower detector optical path 56 and onto the lower detector 22″. As described in the previous embodiment, a demodulated electrical signal (RF_out) is generated at the receiver output 18. This configuration, however, avoids the carrier attenuation resulting from the transmissive optical filters 20′, 20″ of the previous embodiment.

[0032] The reflective filters 20′, 20″ can be the Fabry-Perot type (e.g., Micron Optics, Inc., Atlanta, Ga., tunable filter and controller model no. FFP-TF 1550-050M200-5 for a wavelength of 1550 nm and having a free spectral range of approximately 10 GHz and a finesse of approximately 200). FIGS. 5A and 5B illustrate how the narrow passbands 58, 60 of Fabry-Perot filters transmit a single sideband so that it is removed from the optical signals in the embodiment shown in FIG. 4. Nearly all of the light outside the passbands 58, 60 is reflected back to the optical circulators 50, 52.

[0033] 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 may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.