---
title: "Short note: MASER"
subject: "RF and Microwave Engineering"
chapter: 4
key: "semester_7/rf_and_microwave_engineering:maser-note"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/maser-note
site: BCT Engineering
---

# Short note: MASER

RF and Microwave Engineering · Chapter 4: RF and Microwave Components and Devices

- Asked in: 2080 Baisakh, 3 marks
- Video: [MASER (Basics, Working, Materials, Applications & Uses) Explained](https://www.youtube.com/watch?v=Fpmy5kDNWcM) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/maser-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-4

## Question

Write a short note on MASER.

## Short answer

MASER stands for Microwave Amplification by Stimulated Emission of Radiation. It achieves amplification through population inversion in a material (e.g., ruby) pumped to a higher energy level. Extremely low noise temperature (few kelvin) is obtained by cryogenic cooling, making it ideal for radio telescopes and deep-space receivers.

## Answer

**MASER** stands for **Microwave Amplification by Stimulated Emission of Radiation**. It is a very low-noise microwave amplifier or oscillator that uses stimulated emission from atoms or molecules. C. H. Townes built the first one in 1954, an ammonia maser at 23.87 GHz.

**Principle**
- An atom in an upper energy level E₂ can be pushed down to E₁ by an incoming photon of frequency f = (E₂ − E₁)/h. It then emits a second photon of the same frequency and phase. This is **stimulated emission**.
- Amplification needs **population inversion**, meaning more atoms in E₂ than in E₁. This is obtained by pumping.
- In the common **three-level solid-state maser** (ruby, Cr³⁺ in Al₂O₃), a pump signal at f₁₃ lifts atoms from level 1 to level 3. Level 3 then becomes more populated than level 2, so a signal at f₃₂ is amplified. The levels are tuned by a DC magnetic field (Zeeman splitting).

```text
 E3 ------------  <- pump f13 raises atoms
        | signal f32 amplified
 E2 ------------
 E1 ------------
```

**Features**
- Extremely low noise temperature (a few kelvin), because the active medium is cooled to liquid-helium temperature.
- Narrow bandwidth and low output power. Needs cryogenic cooling and a magnetic field.

**Applications:** front-end amplifiers in radio telescopes and deep-space communication receivers, and atomic frequency standards (the hydrogen maser at 1.42 GHz).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What must be achieved for amplification in a maser?
   A. More atoms in the lower level than in the upper
   B. Population inversion, with more atoms in the upper level, obtained by pumping
   C. Equal populations
   D. Removal of the magnetic field
   Answer: B. Stimulated emission exceeds absorption only with inversion, produced by a pump. (Key point: Population inversion)

2. Why does a solid-state maser have extremely low noise temperature?
   A. It is cooled to liquid-helium temperature
   B. It runs hot
   C. It has wide bandwidth
   D. It uses a transistor
   Answer: A. The active medium is cryogenically cooled, giving noise of a few kelvin. (Key point: Low noise)