Differential current mirror mirrors difference of two refference curents. This means that the output current is equal or proportional to the difference of two input currents.

Why would you use a current mirror as a ciurrent sensor is not clear for me.

In a research paper i have read that combination of current mirror and current comparator can be used as current sensor.

thay rplicate the current using current mirror and then compare it with some reference value to identify the desired value of current.   

Yes, it is possible, however makes no sense. Usualy current is sensed with a low resistance in the current loop. If  you do not want to have resistance in the loop, you can measure magnetic field arround the conductor.  It also depends on how large the current is. uA, ma, A

I do not see any benefit of using current mirror.

Yes thats why i was confused. However i have found another crcuit which senses the current using the low resistance but still they are using the current mirror before comparison. The schematic of this circuit is given in attachement. Discard the other blocks of the schematic. 

Actually my question is that why current mirror is  required?? is theer any other method to sense and compare the currents without using current mirrors?  

It depends what you mean by sensing current. Is it current detector (on/of), amplifier, or you want to measure current. Basicaly you need a resistor, to convert current to voltage, differential amplifier, to amplify this voltage and a comparator, if you want only to detect presence of a current. The diferential amplifier may or may not contain a current mirror. It depents of its implementation.

Actually i want measure the current and then compare it with a standrad value to check whether the circuit is working under the restriction. Moreover if i will use  the op amp then area of chip increases. Actually i have a very strict restriction of area.  

So i have to look into the different implementations of Op amps.

Is there any way to implement the op amp with minimum transistors?? 

Implementation depends on may factors.

What would you like to have as the output? Should output be low impendance? Do you have fixsed values of the current you want to compare to? Do you have dual voltage supply? Should the resistor for sensing current be floating?

Different implementations of op amps are usualy included in IC design tools. Op amps usualy needs very small space on IC, but you do not neccessarly need op amp. This depends on the above factors.

required output should be current ... yes output must have low impedence .... and i have the fixed values of currents for comparison... no it is not necessary to have dual voltage voltage.