some preferred embodiments of the present invention will now be described in detail with reference to the attached drawings . throughout the drawings , the same reference numerals denote like or corresponding parts . [ 0031 ] fig1 is a block diagram showing a preferred embodiment of the terminal device according to the present invention . this device may be used as each onu in an atm - pon system . this terminal device includes a main circuit 10 and a wdm ( wavelength division multiplexing ) coupler 12 optically connected to the main circuit 10 . the main circuit 10 includes a receiving section 2 for receiving an optical signal having a wavelength λ1 ( e . g ., 1 . 5 μm ), a current control circuit 4 , a transmitting section 6 for transmitting an optical signal having a wavelength λ2 ( e . g ., 1 . 3 μm ), and a prebias circuit 8 . the wdm coupler 12 couples the optical input having the wavelength λ1 relating to the receiving section 2 and the optical output having the wavelength λ2 relating to the transmitting section 6 to the same optical path 14 . the receiving section 2 includes a photodetector ( pd ) 16 for converting the optical signal having the wavelength λ1 into a current signal , a preamplifier 18 for converting the current signal output from the photodetector 16 into a voltage signal , and a main amplifier 20 for amplifying the output from the preamplifier 18 to obtain a data output . the transmitting section 6 includes a laser diode ( ld ) 22 as a light source for outputting the optical signal having the wavelength λ2 , and an ld driver 24 for driving the laser diode 22 according to a data input . the current control circuit 4 includes an amplitude detecting circuit 26 for detecting a value corresponding to the amplitude of the optical signal having the wavelength λ1 according to a signal from the receiving section 2 , a drive current control circuit 28 for decreasing and increasing a drive current for the laser diode 22 according to an increase and decrease in the amplitude detected by the amplitude detecting circuit 26 , respectively , and a duty compensating circuit 30 for compensating for the duty of the optical signal to be output from the laser diode 22 according to the amplitude detected by the amplitude detecting circuit 26 . for example , the compensating circuit 30 controls the drive current to be supplied from the ld driver 24 to the laser diode 22 so that the duty of the optical signal to be output from the laser diode 22 is maintained constant . the prebias circuit 8 adds a bias current to the drive current to be supplied from the ld driver 24 to the laser diode 22 so that the oscillation delay jitter of the optical signal to be output from the laser diode 22 is decreased , for example . the optical signal to be output from the laser diode 22 is a burst signal in this preferred embodiment . in this case , the prebias circuit 8 outputs the bias current immediately before the first bit of the burst signal is generated . it is important to increase the dynamic range in the transmitting section 6 from the viewpoint that the extensibility of a network is to be raised . the upper limit of the transmitting power is determined by a maximum rated value in the laser diode 22 , but the lower limit of the transmitting power is an issue to be considered . in the prior art , there is no consideration as to the degree of power reduction that can be attained , and a satisfactory variable range of power cannot be obtained from the viewpoint of the extensibility of a network . the problem arising in lowering the transmitting power is oscillation delay jitter , and the lower limit of the transmitting power is given by a prescribed value of the oscillation delay jitter . the oscillation delay jitter is a phenomenon such that a signal waveform is degraded at the time a bit “ 1 ” rises as shown in fig2 a . that is , when a bit “ 1 ” rises after long continuation of a data sequence of “ 0 ”, the bit “ 1 ” does not normally rise to cause a degradation in signal waveform , because time is required until a carrier density required for emission from the laser diode 22 is reached . further , the prescribed value of the oscillation delay jitter is a degradation quantity of the pulse width of one bit due to the oscillation delay jitter . if the oscillation delay jitter is lower than this prescribed value , normal transmission can be performed . in this respect , the terminal device shown in fig1 employs the prebias circuit 8 to reduce the transmitting power satisfying the prescribed value of the oscillation delay jitter as far as possible . as shown in fig2 b , the prebias circuit 8 preliminarily supplies a bias current to the ld driver 24 immediately before the burst signal is generated , thereby allowing carrier accumulation and normally raising the first bit of the burst signal . [ 0039 ] fig3 is a graph showing the relation between the oscillation delay jitter and the ld drive current . as mentioned above , the oscillation delay jitter can be suppressed by providing the prebias circuit 8 . accordingly , as compared with the case that the prebias circuit 8 is not provided , the curve shown in fig3 can be shifted down , so that a minimum ld drive current satisfying the prescribed value of the oscillation delay jitter can be reduced . in the prior art , there are no measures against waveform degradation in reducing the transmitting power at each onu in the case of system extension , and the dynamic range reducing effect in the receiving section of the olt is therefore insufficient . in contrast , according to this preferred embodiment , the variable range of the transmitting power at each onu can be increased , so that the dynamic range reducing effect in the olt receiving section can be improved to thereby raise the extensibility of a network . further , a sensitivity demand to the olt receiving section can be relaxed by utilizing the reduction of the dynamic range , so that the circuit of the olt receiving section can be easily designed . [ 0040 ] fig4 a is a block diagram of a system in the prior art , and fig4 b is a block diagram showing a preferred embodiment of the system according to the present invention . the system shown in fig4 b is configured by connecting a plurality of onus 32 as subscriber terminals each having the configuration shown in fig1 and a single olt 34 through a star coupler 36 . the star coupler 36 , each onu 32 , and the olt 34 are connected together by optical fiber transmission lines 38 . each onu 32 includes a light source for outputting an optical signal having a wavelength λ2 toward the olt 34 , and the olt 34 includes a light source for outputting an optical signal having a wavelength & amp ; k1 toward each onu 32 . in the prior art shown in fig4 a , a plurality of onus 32 ′ each not adopting the present invention are provided in place of the onus 32 . each onu 32 ′ outputs an optical signal toward the olt 34 with a constant transmitting power . in the case that losses in the optical fiber transmission lines 38 connecting the onus 32 ′ and the star coupler 36 are different from each other , the receiving power of the olt 34 is smaller for the optical fiber transmission line 38 having a higher loss , and is larger for the optical fiber transmission line 38 having a lower loss . as a result , the power reaching the olt 34 reflects the transmission line loss as it stands , and the dynamic range is large . in the system shown in fig4 b , the receiving power of each onu 32 reflects the transmission line loss according to the present invention . by utilizing this feature , the transmitting power of each onu 32 is increased in the case that the receiving power of each onu 32 is small , whereas the transmitting power of each onu 32 is decreased in the case that the receiving power of each onu 32 is large . furthermore , the prebias circuit 8 outputs a bias current immediately before the first bit of the burst signal is generated ( e . g ., within a time period corresponding to two bits ). by performing this control , the dynamic range in the receiving section of the olt 34 can be reduced , and this reduction can be assigned to an increase in the number of subscribers , an increase in transmission distance , or a decrease in transmission distance , for example . this will now be described more specifically . fig5 a ( a ) and 5 a ( b ) illustrate the case of increasing the number of subscribers . in this case , the number of branches from the star coupler 36 toward the onus 32 is increased from 32 to 64 . even when the transmitting power of each onu 32 is increased by 3 db , the power reaching the olt 34 is maintained constant . thus , the number of subscribers can be increased with the receiving power of the receiving section in the olt 34 being constant . fig5 b ( a ) and 5 b ( b ) illustrate the case of increasing a transmission distance . in this case , a certain one of the onus 32 is relocated from an original position shown in fig5 b ( a ) to a far position shown in fig5 b ( b ). by increasing the transmitting power of this relocated onu 32 , the receiving power of the olt 34 can be maintained constant . fig5 c ( a ) and 5 c ( b ) illustrate the case of decreasing a transmission distance . in this case , a certain one of the onus 32 is relocated from an original position shown in fig5 c ( a ) to a near position shown in fig5 c ( b ). by decreasing the transmitting power of this relocated onu 32 , the receiving power of the olt 34 can be maintained constant . in this manner , the extension of a network can be easily realized with the power of an optical signal from each onu 32 to the receiving section of the olt 34 being maintained constant . referring to fig6 there is shown a circuit diagram of the drive current control circuit 28 shown in fig1 . an inverting amplifier is configured by an operational amplifier op 1 and resistors r 1 , r 2 , and r 3 . a voltage input vin from the amplitude detecting circuit 26 ( see fig1 ) is supplied through the resistor r 1 to an inverting input port of the operational amplifier op 1 . a reference voltage vref is supplied to a noninverting input port of the operational amplifier op 1 . the noninverting input port is grounded through the resistor r 3 . the inverting input port and an output port of the operational amplifier op 1 are connected by the resistor r 2 . the gain of this inverting amplifier is given as r 2 / r 1 . by properly setting the values of the resistors r 1 and r 2 and the reference voltage vref , the transmitting power of each onu 32 can be made variable in such a manner that when the amplitude of a received signal is small , the ld drive current is made large , whereas when the amplitude of the received signal is large , the ld drive current is made small as shown in fig7 . the prebias circuit 8 shown in fig1 has a function of decreasing the oscillation delay jitter even when the ld drive current is small . as a result , the dynamic range in the transmitting section 2 of each onu 32 can be made large . when the amplitude of the received signal is relatively small as shown by a point a in fig7 the ld drive current is set to a value giving the upper limit of the power determined by the rated value of the ld 22 , for example . when the amplitude of the received signal is large as shown by a point b in fig7 the ld drive current is set to a value giving the lower limit of the power determined by the prescribed value of the oscillation delay jitter . referring to fig8 there is shown a circuit diagram of the duty compensating circuit 30 shown in fig1 . an inverting input port of an operational amplifier op 11 is grounded through a resistor r 11 . an output voltage vin from the amplitude detecting circuit 26 is supplied to a noninverting input port of the operational amplifier op 11 . the inverting input port and an output port of the operational amplifier op 11 are connected by a resistor r 12 . the output port of the operational amplifier 11 is connected through a resistor r 13 to an inverting input port of an operational amplifier op 12 . a reference voltage vref ′ is supplied through a resistor r 15 to a noninverting input port of the operational amplifier op 12 . the noninverting input port of the operational amplifier op 12 is grounded through a resistor r 16 . the inverting input port and an output port of the operational amplifier op 12 is connected by a resistor r 14 . the output port of the operational amplifier op 12 is connected to the gate of an fet ( field effect transistor ) f 1 . the source of the fet f 1 is grounded , and the drain of the fet f 1 is connected through a resistor r 17 to a duty control circuit . the duty control circuit is incorporated in the ld driver 24 shown in fig1 for example . a noninverting amplifier is configured by the resistors r 11 and r 12 and the operational amplifier op 11 . accordingly , the voltage signal vin corresponding to the receiving power of each onu 32 is amplified . an adder is configured by the resistors r 13 , r 14 , r 15 , and r 16 and the operational amplifier op 12 . this adder functions to add the reference voltage vref ′ to a signal amplified by the noninverting amplifier . a resultant signal obtained by the addition is input into the gate of the fet f 1 . accordingly , a voltage change is converted into a current change , and the duty control circuit is adjusted through a voltage drop at the resistor r 17 . the relation between the gate voltage of the fet f 1 and the duty control circuit is preliminarily measured to obtain a gate voltage corresponding to a duty of 100 %. by setting the values of the resistors r 11 and r 12 in conformity with this voltage , the duty of the optical signal to be output from the laser diode 22 ( see fig1 ) can be maintained substantially constant , thus allowing duty compensation . alternatively , the duty of the optical signal to be output from the laser diode 22 may be maintained substantially constant by decreasing and increasing the bias current according to an increase and decrease in the amplitude of the drive current for the laser diode 22 , respectively . this will now be described more specifically . in general , the oscillation time delay td of a laser diode ( semiconductor laser ) is given by eq . ( 1 ). where τ is the carrier lifetime , and ip , ith , and ib are the drive current , threshold current , and bias current for the laser diode . to make the duty constant , it is sufficient that the antilogarithm in eq . ( 1 ) be constant . in other words , it is sufficient that eq . ( 2 ) hold with the use of a constant k . the relation between the bias current ib and the drive current ip is shown by the graph in fig9 in accordance with eq . ( 3 ). by changing the bias current according to the drive current for the semiconductor laser so that the solid line in the graph is satisfied , the oscillation time delay td can be made constant , thereby obtaining a substantially constant duty . [ 0053 ] fig1 is a block diagram showing a preferred embodiment for obtaining a constant duty . a converting circuit 40 is provided to determine the bias current ib according to the drive current ip so as to satisfy the relation shown in fig9 thereby obtaining a constant duty irrespective of power variations . [ 0054 ] fig1 is a graph showing an experimental result of the relation between the oscillation delay jitter and the ld drive current ip obtained in the preferred embodiment shown in fig1 . this graph substantially corresponds to the graph shown in fig3 . the measurement was made under the conditions of room temperature , a mark rate of ½ , an upstream bit rate of 155 . 52 mbps , and a downstream bit rate of 622 . 08 mbps . the threshold current ith was set to 7 . 6 ma . as a target value ( prescribed value ) of the oscillation delay jitter , 5 % ( 300 ps ) of the pulse width was set . under these conditions , a minimum value of the drive current ip satisfying the target value in the case of applying a prebias ( ib = 3 . 5 ma ) was compared with that in the case of applying a minute bias ( ip = 0 . 13 ma ). as apparent from fig1 , the minimum value of the drive current ip in the case of the prebias is 15 ma , whereas the minimum value of the drive current ip in the case of the minute bias is 28 ma . this reduction corresponds to a reduction of about 4 . 5 db on the basis of optical output power . thus , the advantage of the prebias was confirmed . further , an optical power control experiment was performed by the preferred embodiment shown in fig1 on the basis of the result shown in fig1 . the result of this experiment is shown in fig1 . it was confirmed that the transmitting power of each onu can be changed from + 4 . 1 dbm to − 4 . 2 dbm with a change in the receiving power of each onu from − 29 dbm to − 6 dbm and that the range of duty variations can be suppressed to a substantially constant value by the duty compensating circuit 30 . according to the present invention as described above , the transmitting power of the second terminal device ( onu ) is controlled according to its receiving power . accordingly , the dynamic range of the transmitting power of the second terminal device can be increased . further , the dynamic range in the receiving section of the first terminal device ( olt ) can be reduced , and this reduction can be assigned to the extension of a network . in particular , by adopting a prebias circuit according to a preferred embodiment of the present invention , the transmitting power satisfying a prescribed value of the oscillation delay jitter can be sufficiently reduced . thus , it is possible to provide an optical transmission path system improved in the characteristics of a network , and it is also possible to provide a terminal device applicable to such an optical transmission system . the present invention is not limited to the details of the above described preferred embodiments . the scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention .