Abstract:
A method to identify the wavelength of incoming light is disclosed. The method includes steps to measure a first photocurrent by setting the avalanche photodiode (APD) in a photodiode (PD) mode and a second photocurrent by setting the APD in the APD mode, and to compare a ratio of the two photocurrents with prepared references.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present application relates to a method to identify a wavelength of incoming light to an optical transceiver, and an optical transceiver implementing with the function to identity the wavelength of the incoming light. 
         [0003]    2. Background Arts 
         [0004]    Optical receivers installed within the WDM system have various types and configurations. An optical receiver typically includes an optical receiving unit and a unit to define a wavelength of received optical signal. Such an optical receiver receives light generally multiplexing a plurality of optical signals each having a wavelength specific thereto but different from others. The optical receiving unit receives such multiplexed light and generates a plurality of electric signals each corresponding to the optical signals. The unit to define the wavelength monitors the wavelengths of the multiplexed optical signal. 
         [0005]    One type of optical transceivers, which is often called as a pluggable optical transceiver, is used to be plugged into a port provided in the host system. Not only physical dimensions but electrical specifications to communicate with the host system and optical specifications to communicate with another optical transceiver are precisely defined in a standard for such pluggable optical transceivers. One standard requests that the optical transceiver can receive an optical signal with optical power of −24 dBm. In order to receive such a faint optical signal, an optical transceiver sometimes installs an avalanche photodiode (hereafter denoted as APD) as a light-receiving device because an APD inherently has a function to amplify photo-carriers converted from the faint optical signal. Namely, an APD may generate one or more electron-hole pairs for a single photon. The number of the electron-hole pairs is called as the multiplication factor M, and the parameter M strongly depends on a bias supplied to the APD. 
       SUMMARY OF THE APPLICATION 
       [0006]    One aspect of the present application relates to a method to identify a wavelength of incoming light. The method includes steps of: (a) receiving the incoming light by the APD in the PD mode, where the APD shows the multiplication factor of substantially unity; (b) receiving the incoming light by the APD in the APD mode, where the APD shows the multiplication factor greater than the unity; and (c) identifying the wavelength by comparing the ratio of two photocurrents obtained in the PD mode and the APD mode, respectively, with pre-measured references. 
         [0007]    Another aspect of the present application relates to an optical transceiver that provides an optical receiver, a post-amplifier, and a controller. The optical receiver includes an APD to receive incoming light that has a specific wavelength. The post-amplifier receives data and extracts a clock each contained in the incoming light. The controller measures a first photocurrent output from the APD by providing the first bias that sets the APD in one of the PD mode and the APD mode, a second photocurrent output from the APD by providing the second bias that sets the APD in another of the PD mode and the APD mode, and identifies the specific wavelength of the incoming light by comparing a ratio to two photocurrents with pre-measured references. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]    The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
           [0009]      FIG. 1  is a functional block diagram of an optical a transceiver according to an embodiment of the present application; 
           [0010]      FIG. 2  shows a typical wavelength dependence of a photocurrent generated by an APD when the APD is operated in the PD mode; 
           [0011]      FIG. 3  compares the wavelength dependence of the photocurrent generated by an APD operated in the APD mode with that in the PD mode; 
           [0012]      FIG. 4  shows the wavelength dependence of a response of an APD operated in the PD mode against that operated in the APD mode, where the response is normalized by the ratio at a wavelength of 1470 nm; 
           [0013]      FIG. 5  shows a procedure to define a wavelength of received light carried out within the optical transceiver shown in  FIG. 1 ; and 
           [0014]      FIG. 6  shows responses of photocurrents of an APD against the bias voltage when input optical power Pin is −30 dBm and −20 dBm. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0015]    Next, some preferred embodiments of the present application will be described as referring to drawings. In the description of the drawings, numerals or symbols same or similar to each other will refer to elements same or similar to each other without overlapping explanations. 
         [0016]    First, general characteristics of an APD will be described.  FIG. 6  shows photocurrents (Ipd) output from an APD against a bias voltage applied thereto. As shown in  FIG. 6 , when the applied bias Vapd is relatively small in a region A 1 , the photocurrent Ipd is kept substantially constant against the applied bias but independent of the input optical power Pin. However, when the applied bias becomes large in another region A 2 , for instance, exceeding 28 V, the photocurrent drastically increases. The multiplication factor M is substantially kept in unity (=1) in the former applied bias smaller than 28 V, while, the multiplication factor M drastically increases in the latter bias region greater than 28 V. 
         [0017]    In the explanations presented below, the former region, where the applied bias is smaller than 28 V and the multiplication factor M is unity, is called as that the APD is operated in the PD mode; while, in the latter region where the multiplication factor is greater than unity, the APD is operated in the APD mode. An APD is generally operable in a mode to show the multiplication factor greater than unity; accordingly, the bias supplied to the APD usually exceeds scores of volts. 
         [0018]    Moreover, because the photocurrent output from an APD generally shows a wavelength dependence, the identification of the wavelength of light just received becomes inherently necessary when such an APD is used in the WDM system, in particular, in the CWDM (Coarse Wavelength Division Multiplexing) system. The CWDM system utilizes a wavelength range from 1300 nm to 1700 nm. Sequential procedures, that is, identifying the wavelength of the optical signal received by the APD, processing the photocurrent electrically which depends on the wavelength, and recovering data contained in the received optical signal, become necessary in an optical transceiver. 
         [0019]      FIG. 1  is a functional block diagram of an optical transceiver  1  that has a function to communicate with a host system  100  electrically. The optical transceiver  1  couples with duplex optical fibers to receive the incoming light F 1  and transmit the outgoing light F 2 . The optical transceiver  1  includes an optical receiver Rx, an optical transmitter Tx, a post-amplifier  3 , a driver  4 , a controller  5 , and a memory  6 . The optical receiver Rx includes an APD  2   a  and a trans-impedance amplifier (TIA)  2   b  to convert a photocurrent generated in the APD  2   a  into an electrical signal. The controller  5  includes a bias generator  5   a,  a current detector  5   b,  a resistor  5   c,  and a memory  5   d.  Although the optical transceiver  1  shown in  FIG. 1  provides the memory  6  outside of the controller  5 , the controller  5  integrates the memory  6  with the inner memory  5   d.  In addition, the controller  5  includes the bias generator  5   a,  the resistor  5   c,  and the current detector  5   b  in the arrangement shown in  FIG. 1 ; the controller  5  may push these units externally. 
         [0020]    The optical receiver Rx, which receives the incoming light F 1 , is connected to the post-amplifier  3  and to the controller  5  through the command line L 1   a . Receiving the incoming light F 1 , the optical receiver Rx converts it into a voltage signal to be provided to the post amplifier  3 . 
         [0021]    The post-amplifier  3 , which is provided with the voltage signal from the optical receiver Rx, recovers data and extracts a clock from the voltage signal. The post-amplifier  3  outputs the data and the clock to the host system  100  through a signal line L 1   b . Also, the post-amplifier  3 , which is connected to the controller  5  through the command line L 1   c , provides an alarm named “Loss-Of-Signal” (hereafter denoted as LOS) to the controller  5  through the command line L 1   c . When the incoming light F 1  lowers power thereof less than a preset threshold such that the post-amplifier  3  is impossible to recover data and extract a clock stably or securely, the post-amplifier  3  sets LOS to the controller  5 . 
         [0022]    The optical transmitter Tx outputs the outgoing light F 2 . The optical transmitter Tx, receiving a modulation signal from the driver  4 , converts the modulation signal into the outgoing light F 2  and outputs the light F 2  to the optical fiber. 
         [0023]    The driver  4 , which is electrically connected to the host system  100  through the control line L 2   a  and the controller  5  through the command line L 2   b,  receives the modulation signal from the host system  100  through the signal line L 2   a  and drives the optical transmitter Tx by providing the modulating signal to, for instance, a light-transmitting device such as semiconductor laser diode (hereafter denoted LD) installed in the optical transmitter Tx. The driver  4  receives a command TxDISABLE from the controller  5  through the command line L 2   b,  which forcibly stops the LD from emitting the outgoing light F 2 . 
         [0024]    The controller  5  executes a firmware stored in the memory  5   d  to control the optical transceiver  1  in normal operations Also, the controller  5  stores statuses in the optical transceiver  1  detected thereby in the memory  5   d.    
         [0025]    The controller  5  is connected with the host system  100  through the control line L 3 ; while, the controller  5  is connected to the optical receiver Rx through the command line L 1   a  to provide the bias for the APD generated in the bias generator  5   a;  receives the alarm LOS from the post-amplifier  3  through the command line L 1   c ; and provides the command TxDISABLE to the driver  4  through the command line L 2   b.  Also, the controller  5  stores status parameters in the optical transceiver  1  within the memory  6 , and reads thus stored parameters from the memory  6 . 
         [0026]    The optical receiver Rx will be further described in detail. The optical receiver Rx includes the APD  2   a  and the TIA  2   b.  The APD  2   a  in the anode thereof is connected to the TIA  2   b  to transfer the photocurrent generated by the APD  2   a.  The TIA  2   b  converts the photocurrent into a voltage signal to provide to the post-amplifier  3 . The APD  2   a  may be made of semiconductor material having an optical sensitivity for light with wavelengths from 1450 to 1650 nm, typically, the APD  2   a  includes a light-sensing layer (an active layer) primarily made of InGaAs. 
         [0027]    The APD  2   a  in the cathode thereof is supplied with the bias voltage Vapd reaching several scores of volts, which is generated in the bias generator  5   a,  through the signal line L 1   a . Moreover, the controller  5  detects an average of the photocurrent by the current detector  5   b.    
         [0028]    Specifically, because of the fundamental theory of the current continuity, the photocurrent caused in the cathode of the APD  2   a  is equal to the photocurrent caused in the anode thereof. The cathode of the APD  2   a  provides a low-pass filter constituted by a capacitor and a resistor, then, the signal obtained in the cathode of the APD  2   a  corresponds to an average power of the incoming light F 1 . The current detector  5   b  may detect this average photocurrent as a voltage drop induced in the resistor  5   c.  That is, the output of the current detector  5   b  corresponds to the average power of the incoming light F 1 . 
         [0029]    Next, characteristics of the APD  2   a  will be described. An APD may generate a photocurrent reflecting incoming light, but the magnitude of the photocurrent depends on a wavelength magnitude of the optical signal, and a bias voltage.  FIG. 2  shows a typical wavelength dependence of a photocurrent generated by an APD when the APD is operated in the PD mode, that is, the APD is supplied with a bias such that the APD shows the multiplication factor of the unity. The photocurrents shown in  FIG. 2  are measured under a condition that an ambient temperature is 25° C. and the optical power is −20 dBm. The characteristics of the APD, in particular, the wavelength dependence thereof, inherently depends on materials constituting the APD. Almost all APDs are made of III-V compound semiconductor materials, in particular, in order to show a substantial sensitivity in infrared wavelengths, III-V compound semiconductor materials with the fundamental absorption edge in an infrared wavelength and the type of the direct gap semiconductor is often used. Such semiconductor materials are typically GaAs, InP, InAs and so on. These semiconductor materials show the absorption coefficient abruptly increasing from substantially 0 to 10 4 ˜10 5  /cm at the fundamental absorption edge in an infrared wavelength and gradually increasing in shorter wavelengths. Another semiconductor material InGaAs, which is widely used as a light-receiving device in wavelengths around 1.3 and 1.55 μm, has characteristics similar to those above described. In an example shown in  FIG. 4 , the APD in a light-sensitive layer thereof has the fundamental absorption edge Eg located at 1.63 μm, and substantially transparent in wavelengths longer than 1.63 μm. 
         [0030]    This means that the APD generates substantially no photocurrent for light with wavelengths longer than 1.63 μm. The present embodiment relates to wavelengths shorter than that corresponding to the fundamental absorption edge of the material constituting the APD, in particular, the material of the light-sensitive layer. 
         [0031]      FIG. 3  shows characteristics of the photocurrent of the APD  2   a  in wavelengths shorter than that corresponding to the fundamental absorption edge as varying the bias supplied thereto, namely, as varying the multiplication factor M. The horizontal axis shows a wavelength, while, the vertical axis corresponds to the photocurrent by an arbitrary unit. In  FIG. 3 , the current detector  2   b  measured these cathode currents through the signal line L 1   a . The photocurrents are normalized at the wavelength of 1470 nm. Behavior G 1  corresponds to the PD mode, namely the multiplication factor M of the unity; while, the other behavior G 2  corresponds to the APD mode where the bias of 40 V was supplied to the APD  2   a  to set the multiplication factor M equal to 8. As shown in  FIG. 3 , the decrease of the photocurrents as increasing the wavelength in the APD mode shown by the behavior G 2  exceeds that in the PD mode shown by the behavior G 1 . 
         [0032]      FIG. 4  shows the wavelength dependence of a ratio of two behaviors, G 1  and G 2 , shown in  FIG. 3 , normalized at the wavelength of 1470 nm. That is, the vertical axis denotes the ratio K(λ) calculated by: 
         [0000]        K (λ)= c (λ)/ c (1470 nm),
 
         [0000]    where c(λ) is given by: 
         [0000]        c (λ)= I   M=8 (λ)/ I   M=1 (λ),
 
         [0000]    where I M=8 (λ) is the photocurrent of the APD obtained in the APD mode with the multiplication factor of 8, while, I M=1 (λ) is that obtained in the PD mode. A parameter c(1470 nm) is: 
         [0000]        c (1470 nm)= I   M=8 (1470 nm)/ I   M=1 (1470 nm). 
         [0033]    From relations above described, measuring the photocurrents at respective biases each setting the APD in the PD mode (M=1) and the APD mode (M=8), the optical transceiver may identify the wavelength of the incoming light F 1  currently detected by the optical receiver Rx. Specifically, storing parameters D shown in  FIG. 4  into the memory  6  at respective wavelengths, then detecting two photocurrents measured by the current detector  2   b  at respective modes, namely, M=1 and M=8, calculating the ratio cl of thus measured two photocurrents, evaluating the ratio K 1  (=c 1 /c(1470 nm)), and comparing thus calculated ratio K 1  with parameters D stored in the memory  6 , the optical transceiver may identify the wavelength of the incoming light  6  currently received by the optical receiver Rx. 
         [0034]    The identification of the wavelength  2  of the incoming light F 1  may be carried out at the initializing procedure of the optical transceiver  1 . That is, the identification may be performed when the optical transceiver  1  first receives the incoming light F 1 . 
         [0035]      FIG. 5  shows a flow chart of an algorithm to identify the wavelength of the incoming light. The optical transceiver  1  first confirms by the controller  5  at step S 1  that the optical transceiver  1  practically receives the incoming light F 1 . Allocating an address for a flag in the memory  5   d,  the transceiver  1  sets this flag, which is called as the reset flag, during initializing procedures when the optical transceiver  1  is first set on the host system  100 . The initializing procedures are often called as the power-on reset. When the optical transceiver  1  receives the incoming light F 1  during the reset flag is set, the optical transceiver may decide that the incoming light F 1  is first received and the process to identify the wavelength of the incoming light F 1  is necessary to be carried out. 
         [0036]    The controller  5  further confirms in step S 1  that the incoming light F 1  is not optical noises by referring to the alarm LOS through the signal line L 1   c . When the alarm LOS is set, the controller  5  decides which modes set the alarm LOS, namely, one is a case where the amplitude of the incoming light F 1  itself is less than the preset threshold, and the other is a case where the post-amplifier  3  is impossible to recover the data and extract the clock in spite of the fact that substantial amplitude is detected by the optical receiver. 
         [0037]    After completing step S 1 , two procedures of steps S 2  and S 3  are performed. The step S 2  sets the bias supplied to the APD  2   a  such that the APD  2   a  operates in the PD mode, namely, the multiplication factor M=1. The bias Vapd set to the APD  2   a  in the PD mode is generally a power supply voltage supplied to front end circuits such as the TIA  2   b  in the optical receiver Rx, which is typically 3.3 volts. The controller  5  obtains the photocurrent I(M= 1 ) as irradiating the APD  2   a  by the incoming light F 1 . 
         [0038]    The step S 3  sets the bias to a value at which the APD  2   a  is operated in the APD mode to show the multiplication factor M to be equal to, for instance, 8. Such a bias voltage in the APD mode sometimes reaches a score of volts, for instance, 40 volts. Then the controller  5  may get another photocurrent I(M=8) as the APD  2   a  receives the incoming light F 1 . 
         [0039]    An optical transceiver, or a bias generator  5   a  may generally vary the bias Vapd supplied to the APD  2   a  depending on, for instance, an ambient temperature of the APD  2   a,  the magnitude of the incoming light F 1 , and so on. Specifically, the bias Vapd is adjusted such that, when the APD  2   a  receives incoming light with amplitude in a preset range, the APD  2   a  generates a photocurrent with magnitude in a preset range by feeding the photocurrent back to the bias generator. 
         [0040]    On the other hand, the present arrangement fixes the bias Vapd in a preset and constant value, for instance 40 V, in the initializing procedures. Fixing the bias Vapd, the photocurrent output from the APD  2   a  simply depends on the amplitude of the incoming light F 1 . Then, evaluating the ratio of the photocurrents in respective modes, I M=8  and I M=1 , the dependence of the photocurrent ratio on the wavelength may be evaluated. 
         [0041]    The two modes, the APD mode and the PD mode, are sequentially switched; then, the magnitude of the incoming light F 1  possibly fluctuates. The controller  5 , when the fluctuation of the magnitude of the incoming light F 1  between the two modes is detected, may compensate the fluctuation. For instance, switching between two modes as PD 1 →APD 1 →PD 2 →APD 2 → . . . PD i →APD i → . . . by varying the bias Vapd and evaluating ratios of two photocurrents sequentially as follows: 
         [0000]        I   M=8   n   /I   M=1   n−1 , 
         [0000]        I   M=8   n   /I   M=1   n , 
         [0000]        I   M=8   n+1   /I   M=1   n , 
         [0000]        I   M=8   n+1   /I   M=1   n+1 , 
         [0000]    where I M=8   i  and I M=1   i  mean the photocurrents in the APD mode and the PD mode detected at i th  measurement, respectively. The controller  5  may determine the ratio when ratios obtained by the above procedures converge. That is, the controller  5  iterates steps S 2  to S 5  until the calculated ratio of two photocurrents converges. 
         [0042]    The controller  5  compares the ratio evaluated by the above procedures with data D pre-stored in the memory  6  to identify the wavelength λ of the incoming light F 1 , and informs thus identified wavelength λ to the host system through the signal line L 3 . Then, the optical transceiver  1  practically operates to receive and transmit optical signals. 
         [0043]    According to an optical transceiver thus described, the wavelength of the incoming light F  1  may be identified during the initializing procedures performed by the controller  5  without preparing additional optical and electrical components. In an alternative, the controller  5  may carry out the wavelength identification procedures when the optical transceiver  1  first receives the incoming light F 1  after the power-on reset initializing. The identified wavelength is provided to the host system  100 , the host system  100  may allocate other wavelengths, grids, or channels, to other optical transceivers, which may facilitate the effective operation of the optical communication system. 
         [0044]    The procedures to identify the wavelength X, may be carried out not only during the power-on reset but at a time when the optical transceiver  1  first receives light after resetting the status LOS. The status LOS is set when an optical connector is once released from the optical transceiver  1  even when the optical transceiver  1  is set on the host system  100 . A combination of a transmitter optical transceiver and a receiver optical transceiver is probably revised, which means that the wavelength of the incoming light shifts to a new wavelength from a last one. In such a case, the controller  5  sets the status LOS because of the disappearance of the incoming light FL Then, after establishing the incoming light, the controller  5  negates the status LOS and begins the wavelength identification process because the wavelength of the incoming light is possibly shifted from the former one. Thus, not only during the power-on reset but being triggered by the reset of the status LOS, the sequential procedures to identify the wavelength of the incoming light is preferably carried out. Because the optical transceiver  1  may identify the wavelength of the incoming light F 1  without preparing additional devices, the wavelength identification procedures may be flexibly carried out even after the status LOS is reset. 
         [0045]    Although the present invention has been fully described in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, it is to be understood that various changes and modifications may be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims.