The present invention relates to an optical attenuator and, more particularly, to an optical attenuator which utilizes a thermal bimetal to provide attenuation by adjusting the path length of light travelling through the bimetal. Some fiber optic communication systems include a network element having a transmit optical power adjustment capability and a corresponding receive optical power adjustment capability. The power adjustment is utilized to adjust or offset the effect of temperature variations in the fiber in a network segment. The temperature variations in the fiber can cause attenuation losses and therefore affect the operation of a link in the system. The transmit optical power adjustment capability compensates for temperature induced fiber attenuation by automatically adjusting the optical power launched into the fiber. Present transmit optical power adjustment mechanisms utilize a thermistor mounted to the optical fiber or to the heat sink that dissipates the heat generated by the thermistor. The difference in temperatures between the fiber and the thermistor is measured to determine the extent of fiber temperature variations or, conversely, how much temperature control is required. A control circuit is utilized to sense the temperature differential between the fiber and the thermistor. Based upon the temperature differential, the control circuit determines the required adjustment in the optical power launched into the fiber and sends a control signal to a device which adjusts the transmit power. This allows the transmit power to be adjusted in response to fiber temperature variations. An example of a thermistor based optical power adjustment mechanism is disclosed in U.S. Pat. No. 5,923,760 issued to Duffner, et al. The mechanism of the Duffner, et al. patent uses a feedback loop having a thermistor sensor that senses the temperature of an optical fiber. The sensing circuit also monitors an output signal from the transmitter. The signals are sent to a control circuit which determines a difference in magnitude between the two signals and determines a required change in the transmit power based upon the magnitude of the difference signal. A control signal generator outputs a control signal to an attenuator element which adjusts the attenuation level in the optical fiber. A problem with the optical power adjustment mechanisms of the prior art is that they do not provide rapid response to changes in the temperature of the fiber. More specifically, since there is a finite time for the temperature of the thermistor to change and the thermistor only senses the temperature of the entire network segment, there is no assurance that the thermistor will sense a temperature variation in the fiber before a change in power is initiated by the mechanism. This can be a significant disadvantage in optical systems which use a feedback loop to control the temperature of a network segment to a desired temperature. Accordingly, there is a need for an optical power adjustment mechanism that can rapidly adjust the optical power launched into an optical fiber without initiating changes in power which could overshoot the target or steady state temperature of the fiber. There is also a need for an optical power adjustment mechanism that can rapidly adjust the optical power launched into an optical fiber without an undesirable lag in initiating changes in power which would affect the temperature of the fiber. The present invention is directed to these needs. In accordance with the present invention, an optical fiber is monitored with an optical sensor for the temperature differential between the optical fiber and a heat sink connected to a mechanism which adjusts the temperature of the fiber. The optical sensor has at least a first optical sensor for detecting light from the fiber. A control circuit is coupled to the optical sensor and the heat sink. The control circuit receives signals from the optical sensor which represent the output optical power from the fiber and the temperature of the fiber. The control circuit determines a difference between the output optical power and a target optical power value based upon the temperature differential and generates a control signal. The control signal is applied to the mechanism which adjusts the temperature of the fiber. The control signal causes the mechanism to adjust the temperature of the fiber toward the target value until the temperature difference between the fiber and the heat sink is substantially zero. When the temperature difference between the fiber and the heat sink is substantially zero, the control signal also causes the mechanism to adjust the optical power launched into the fiber to the target value. In accordance with the preferred embodiment, a mechanism is provided to drive a set of bimetal fins through which a set of wires are positioned in parallel to form a heat sink for a thermistor. The temperature of the bimetal fins is adjusted by changing the current flowing through the wires of the set. The wires of the set are connected to a thermistor. The thermistor is mounted on a substrate which is secured to the optical fiber. The thermistor measures the temperature of the bimetal fins and sends a signal representative thereof to the control circuit. The control circuit sends a control signal to the mechanism which changes the current flow through the wires of the set. By varying the current flow through the wires, the temperature of the bimetal fins is changed which, in turn, changes the temperature of the thermistor mounted on the substrate and changes the attenuation of an input signal sent into the fiber by a transmitter. The mechanism is preferably controlled by a current that is generated by a digital control circuit. The current is directed to a driver for adjusting a set of current switches which are connected in parallel with the bimetal fins. The current switches are controlled by a feedback circuit to adjust the current flow in the current switches to change the temperature of the bimetal fins. This allows the control circuit to adjust the temperature of the fiber as well as the attenuation of the input signal applied into the fiber by the transmitter. The thermistor is mounted on a substrate which is secured to a connector that has a light input and a light output. The substrate is secured to the fiber, such as a fusion spliced fiber, by attachment to an adhesive between an end of the fiber and the connector. The use of bimetal material to control the temperature of an optical fiber to provide a variable attenuator is advantageous in that it allows a temperature adjusted variable optical attenuator to be used in a number of environments. First, the attenuator can be used in a network in which the temperature of an optical fiber in the network changes. Second, the attenuator is ideal for controlling the temperature of an optical fiber in a network which is not heated and is in direct contact with the earth. Third, the attenuator can be used to control the temperature of optical fibers in a system having a fiber optic cable that is at least partially buried. The use of the present variable optical attenuator also allows for a more efficient system that reduces the required amount of power to operate in an optical system. The temperature compensation loop is able to quickly reach a target temperature and then keep the temperature at the target temperature. When the attenuator reaches a steady state at the target temperature, the transmission link has a desired power level. Since the system does not have to heat or cool as much when the attenuator reaches the steady state at the target temperature, less power is required to control the system in order to maintain the steady state target temperature. Thus, not only is the efficiency of the system increased because less power is required to control the system to achieve a particular steady state target temperature, but the power supply can be designed with a lower capacity. The use of the variable optical attenuator also reduces the amount of signal distortion. For example, if the variable optical attenuator is used in a variable length loop, attenuation of input optical signals into an optical fiber will result in the generation of an optical overshoot. The amount of overshoot of the input optical signals is affected by the amount of attenuation. A reduction in attenuation to avoid or reduce the overshoot in the received input optical signals will increase the transmission level in a variable length loop. If the variable optical attenuator is used in an optical-electric-optical (OEO) conversion loop, the input optical signals from an input optical fiber can have an undesirably high overshoot due to over-attenuation of the input optical signals. By reducing the amount of attenuation, the overshoot is reduced