The device used for detecting optical signal is 

Options

A. Zener diode
B. photodiode
C. LED
D. junction diode

Explanation

To answer the question "The device used for detecting optical signal is," we can follow these steps:

### Step 1: Understand the Options
We need to evaluate the options given for the device that can detect optical signals. The options include Zener diode, junction diode, photodiode, and LED.

### Step 2: Analyze Each Option
- **Zener Diode**: This device is primarily used as a voltage regulator. It maintains a specific voltage across a load but does not detect optical signals.

- **Junction Diode**: This is a basic diode used for various applications, but it is not specifically designed for detecting optical signals.

- **Photodiode**: This is a specialized type of diode that is designed to detect light (optical signals). It is transparent and can convert light energy into electrical energy, making it suitable for this purpose.

- **LED (Light Emitting Diode)**: While it emits light when current passes through it, it does not detect optical signals.

### Step 3: Identify the Correct Answer
From the analysis, the photodiode is the only device specifically designed to detect optical signals. Therefore, the correct answer is **Photodiode**.

### Step 4: Explain How Photodiode Works
A photodiode operates by allowing light to fall on it, which excites electrons from the valence band to the conduction band in a semiconductor material. This movement of electrons generates a current, which can be measured. Photodiodes are typically used in reverse bias to enhance sensitivity to small changes in light intensity.

### Final Answer
The device used for detecting optical signals is **Photodiode**.

---