Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2002-08-27
2004-01-27
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S513000
Reexamination Certificate
active
06683490
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a constant-current generating circuit and more particularly to a temperature dependent constant-current generating circuit applied to a feed forward type stabilized driving circuit for keeping a light emission characteristic of a light output device composed of a laser diode as a light source such as a optical transmission device or an optical data link.
Optical communications and optical data links have been suddenly widespread in recent years. In the optical transmission device used for these uses generally, a laser diode (LD) is directly modulated in intensity so as to generate a optical signal and the optical signal is transmitted via a transmission medium such as an optical fiber or by propagating through the free space. Particularly, in a transmission module used for a optical data link such as not only a subscriber system such as FTTH (fiber to the home) for use of optical communication at each home but also giga-bit Ethernet, IEEE1394, and optical wiring, the following method is adopted as a method for directly modulating the intensity.
The LD is kept at a DC bias current less than the laser oscillation threshold value in a state where an optical signal is OFF. However, in a state where a optical signal is ON, the LD is brought to a condition where a perfect laser oscillation is obtained, applying a pulse current with an amplitude enough to obtain necessary output intensity, thereby making the intensity ratio of a optical signal in ON and OFF state as large as possible.
When a signal to be transmitted is at a comparatively low frequency, zero bias driving is used, in which the bias current in the OFF state of the LD is perfectly 0. However, when the ON-OFF modulation frequency is increased, simple application of zero bias driving is difficult for the following reason. Assuming a life of carrier decided by the LD to be used as &tgr;, a threshold value of a current in the LD as Ith, the DC bias current to be continuously applied even while the LD is not in operation as Ib, and the driving pulse current amplitude while the LD is in operation as Ip, it is known that the delay time Td from the time when the current is injected to the time when the laser oscillation of the LD takes place is given by the following formula.
Td=&tgr;×In
(
Ip
/(
Ip+Ib−Ith
)) (1)
Since &tgr; is generally on the order of nano seconds, it is important to reduce the value of the logarithm term in the above formula to a value as small as possible such as 0.1 or less, when a signal transmission speed of 100 Mb/s or more is required. To satisfy Ib=0 which is a perfect zero bias condition, as well as to realize a value of logarithm term of 0.1 or less, it is necessary to reduce the ratio Ip/Ith to 0.1 or less, that is, to make the value of Ip extremely larger than 10 times of the value of Ith. In other words, the pulse driving current amplitude must be set to a value larger than the value for obtaining the necessary laser intensity amplitude. A problem thus arises that the current driving capacity of the LD driving circuit must be made higher than the value to be required at its minimum so as to obtain the necessary transmission optical signal intensity and that, as a result, the power consumption is increased at the same time.
On the other hand, in the pseudo-zero bias driving system, in which the DC bias current Ib is always applied which is slightly smaller than the threshold value Ith, the ratio Ip/(Ith−Ib) can be set easily to 10 or more, which is more advantageous even if Ip itself is not made large so much. Namely, when the pseudo-zero bias driving system is used, the delay time is reduced, and the high frequency operation is ensured, as well as a large light ON/OFF ratio is obtained and low power consumption can be realized at the same time.
However, a problem still remains that it is difficult for the pseudo-zero bias driving system to control Ib. The reason is that it is known that the threshold value of current Ith at an optional temperature Ti is expressed by the following approximate value, using an intrinsic characteristic temperature T
0
of the LD to be used and a threshold value Is when the reference temperature T=Ts:
Ith=Is
×exp((
T−Ts
)/
T
0
) (2)
Exhibiting a characteristic that Ith greatly changes with non-linearly for a temperature change. For example, the value T
0
of an InP series LD is several tens to 100, so that the threshold change is close to several times to 10 times for a temperature change of 100 degrees. Further, even in a case of a GaAs series LD which is knnwn to have a characteristic of comparatively low temperature dependence within the range from the room temperature to about 70° C., a constant term of Ic may be added to the right side of Formula (2) to increase accuracy in the approximation within the temperature change ranging from −40° C. to 100° C. It is known that thus compensated value of T
0
shows the same value as that of an InP series element. In consideration of the above, it is essential for the DC bias generating circuit itself to have large temperature dependence in the same way as with Ith to make the value of Ib for realizing a pseudo-zero bias follow Ith and keep the difference between them to be substantially constant.
In the art prior to the disclosure of Japanese Patent Application Laid-Open 11-103108 applied by the applicant and shown in
FIG. 1
, a simple bias current generating circuit which can accurately follow temperature changes of the threshold current Ith and can be applied to LDs having various different characteristics was not realized. For example, a system for searching for the inflection point, at which a DC bias current is fixed, in the neighborhood of the threshold current by checking the differential value of a DC bias current, and a system for monitoring the intensity of actually emitted light of the LD, which is fed back to a DC bias are known as a compensation system of threshold bias current of LD. These systems require a large-scale detecting and feed backing circuit, so that it is almost impossible to apply them to uses requiring a compact one-chip IC such as LD driving circuits for the optical data link.
On the other hand, it is known that not only the threshold value Ith but also the light intensity emitted of the LD have temperature dependent characteristics, which is expressed by an exponential function decreasing with temperature, assuming the characteristic temperature T
0
′ as a constant. Since the value of T
0
′ is large compared with T
0
and is generally several hundreds, temperature compensation of the light intensity emitted is often required even though it changes slightly unlike the case with the threshold value. In the conventional optical communication, an APC (automatic power control) circuit for keeping the light intensity emitted from the laser constant is used to keep the magnitude of a optical transmission signal constant and to prevent deterioration in quality of the signal transmitted. A large-scale APC circuit for monitoring a part of output of the LD by a phase detector (PD) and feeding it back is generally used to realize severe control by active feedback.
Further, due to an improvement in uniformity and stability of physical characteristics of LD, in recent years, a feed forward type stabilizing circuit is used on a assumption that the temperature dependent characteristic of LD is regarded as almost constant. Namely, as a method for a simple temperature compensation of the light intensity emitted from LD, stabilization using a control system for applying passive feed forward is adopted.
Such a temperature compensation system of light intensity emitted from the LD in the feed forward type APC circuit is exemplary shown in Japanese Patent Application Laid-Open 3-21493.5 and Japanese Patent Application Laid-Open 8-139410. Following systems have been designed by confirming the characteristics of LD beforehand.
(1) a system for making ro
Cunningham Terry D.
Hogan & Hartson LLP
Kabushiki Kaisha Toshiba
Tra Quan
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