Question: I know that there are clamp meters to measure currents for electrical wires externally without almost not interfering the circuit.

I was wondering whether in the case of an optical fiber(single mode silica optical fiber), is there a technique or device/component which can detect the laser light power externally or wrapping a sensor around ect?

Answer: 
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> What determines the current it produces?
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A computer running a maximum power point tracking (MPPT) algorithm. If the turbine starts slowing down too much, the computer will reduce the amount of power it tries to export. If the turbine starts spinning too fast, it will generate more power.

The output power is determined by adjusting the "on" times of the output transistors.

Solar inverters do much the same thing, but with a different algorithm.

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> But does it have something to do with the "impedance" of the grid?
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No. the grid impedance should be very low.
Several restrictions take place when controlling a wind turbine.
Normally the DC bus is always kept at a constant voltage, since that voltage is not only the means of pumping power into the grid, but it's also the voltage supplied to the internal circuitry needed to control microcontrollers, SBCs, BMSs (because when grid connection is lost you MUST have a means to stop the machine or at least hold it spinning for an unexpected 10-second grid loss and later restablishment), slip ring data transmission between rotating and non-rotating parts (\$\textbf{e.g.}\$ Nacelle and Hub) and the list goes on.
The machine as mentioned in the OP is a PMG, from what I assume is a PMSM (Permanent-Magnet Synchronous Machine).

The whole operation relies on what the grid is demanding.
The AC-DC section takes the induced voltage from the generator, rectifies it and charges the capacitor.
The inverter is the DC-AC section that controls power sent to the grid and links the converter to the grid transformer through the LC filter presentend in the picture. Such filter (especially indcutors) play a really important part in the machine operation since they tend to be bulky, very expensive, dissipate a LOT of heat and are critical to the desired operation.

When more power is needed, more current is demanded from the generator, which tends to slow the rev/s, and thus a upper limit is set not to damage semiconductors due to overcurrent. An algorithm sets the yaw angle to face more or less wind, thus increasing or decreasing mechanical output power.

The dangerous operation takes place when the machine is \$\textbf{not}\$ transfering energy to the grid, because for a constant wind flow, the blades will still tend to rotate and since there's no (or little) power demand no (or a small) current will be flowing to the grid. This event makes the capacitor voltage increase until a limit where the machine will start to increase its rotating speed, which may mechanically damage the whole structure and even be set on fire due to overspeed.
To avoid such tragedy, the pitch system activates (a blade may rotate about its own axis) and reduces the angle by which blades are relative to the wind flow direction, towards point where the machine will stop completely.

With respect to the inverter design, transistors may usually be IGBTs or SiC MOSFETs, and the operation is pretty much like a three-phase DC-AC converter. A synchronization algorithm is necessary to keep both grid and converter current synchronized, \$\textbf{e.g.}\$ a PLL. Many control techniques may be chosen to control the output current. For instance a PRes (Proportional - Resonant) controller will keep track of measured current and set the value to a predefined sinusoidal reference. This sinusoidal reference may be compared to a triangular waveform, from which the high- and low-side transistors take their duty cycle times from.

By and large this might give you an overview of how OP's wind turbine operates.