The Difference Between Narrow Linewidth Lasers And Single-Frequency Lasers
In the field of laser technology, “narrow linewidth” and “single frequency” are two key characteristics that are often mentioned simultaneously. They all sound related to the “purity” of lasers, so many people tend to confuse the two. However, what they describe are actually two different dimensional characteristics of the laser. Understanding their differences is of vital importance for scientific research, industrial applications and equipment selection.
This article will clearly explain the technical definitions of narrow-linewidth lasers and single-frequency lasers for you, dissect their core differences and connections, and ultimately guide you on how to make the right choice based on practical applications.
Understand Two Fundamental Concepts – Line Width And Longitudinal Mode
To distinguish between narrow linewidth and single frequency, it is first necessary to understand two basic concepts: linewidth and longitudinal mode.
Laser linewidth is a key indicator for measuring the purity or monochromaticity of laser frequency. You can understand it as the degree of concentration of laser energy on the frequency coordinate. The laser’s frequency component is more concentrated, its monochromaticity is improved, and its coherence is strengthened when the line width is smaller. Typically, frequency units like kHz or MHz are used to measure line width.
The physical length of the laser resonator determines the laser’s longitudinal mode. Each frequency is a longitudinal mode, and there may be more than one frequency that satisfies the cavity’s resonance requirements. As a result, the laser can produce light at various frequencies and oscillate concurrently in multiple longitudinal modes.
We can investigate narrow-linewidth lasers and single-frequency lasers further after comprehending these two points.
Narrow-Linewidth Laser
Narrow-linewidth lasers, as the name suggests, have the core feature of having an extremely narrow spectral linewidth. The design and optimization objective of this type of laser is to compress the linewidth of the laser as much as possible and enhance its coherence.
To determine whether a laser is a narrow-linewidth laser, the main focus is on its linewidth parameters, such as whether it reaches the kHz or even sub-khz level. The narrower the line width, the longer the coherence length of the laser, and the better its performance in applications such as interference and sensing.
It is worth noting that a narrow-linewidth laser may operate on only one longitudinal mode, but it can also work simultaneously on several longitudinal modes. As long as the linewidth of each longitudinal mode is narrow enough, it still falls within the category of narrow-linewidth lasers.
Single-Frequency Laser
The definition of a single-frequency laser is even stricter and more absolute. Its core requirement is that the laser must operate on only one longitudinal mode at all times and completely suppress oscillations in all other longitudinal modes.
The key indicator for judging whether a laser is a single-frequency laser is the edge-mode rejection ratio. This parameter represents the ratio of the strength of the main longitudinal mode to that of the strongest side mode (other longitudinal modes), and it is usually required to be higher than 40 dB or even 50 dB. The higher the SMSR value, the higher the degree of “single frequency” and the purer the mode. Of course, as a laser operating in a single longitudinal mode, the linewidth of a single-frequency laser is naturally very narrow.
It can be said that while single-frequency lasers pursue a narrow linewidth, they also impose the strictest regulations on the number of working modes.
Core Differences And Connections
To present the relationship between the two more intuitively, we make a comparison through the following table:
| Characteristic dimension | Narrow-linewidth laser | Single-frequency laser |
| Core definition | Emphasizing linewidth performance means that the spectral width of the laser is extremely narrow | Emphasizing the number of modes refers to the laser operating in a single longitudinal mode |
| Focus on key points | The degree of “narrowness” (such as kHz, MHz) | “Single” mode, focusing on the edge-mode rejection ratio (SMSR) |
| “Technical category” | A broad category of technologies | A stricter and more ideal state |
| Mode status | It can be a single longitudinal mode or a few longitudinal modes (but each line width is very narrow) | It must and can only work in a single longitudinal mode |
| Key parameters | Linewidth, coherence length | Edge mode rejection ratio (SMSR), line width |
| Mutual relationship | A single-frequency laser must be a narrow-linewidth laser | Single-frequency is a special type of narrow-linewidth laser |
By comparison, we can draw a core conclusion: all single-frequency lasers are narrow-linewidth, but not all narrow-linewidth lasers are single-frequency. Single-frequency lasers are the most demanding and high-performance “members” in the “big family” of narrow-linewidth lasers.
Application scenarios and how to choose
After understanding the technical differences, how should we make a choice based on actual applications?
In the following scenarios, strict single-frequency lasers usually need to be given priority:
- High-precision spectroscopy: It requires an absolutely single frequency to scan and detect the resonance lines of atoms or molecules. Any side mode will cause measurement errors.
- Cold atom/ion trap physics: The experiment has extremely high requirements for the frequency noise and stability of the laser. Even the slightest mode jump can disrupt the experiment.
- Precision interferometry: When conducting long-distance and high-precision measurements, extremely long coherence lengths and absolutely stable frequencies are required. Single-frequency characteristics are the fundamental guarantee.
In the following scenarios, high-performance narrow-linewidth lasers (not necessarily demanding the ultimate SMSR) may be sufficient:
- Coherent optical communication: The system has strict requirements for the linewidth of the laser to ensure the receiving sensitivity, but the requirements for the purity of the mode are relatively loose.
- Distributed optical fiber sensing: such as the Φ-OTDR system, its performance is mainly affected by the laser linewidth and is not sensitive to the presence or absence of weak edge modes.
When making a choice, you can ask yourself a key question: Can my application tolerate extremely weak other modes in the laser, or possible minor mode transitions?
- If the answer is negative, then it is necessary to strictly examine the edge-mode rejection ratio index and select a high-performance single-frequency laser.
- If the answer is affirmative or the impact is not significant, then you can focus on parameters such as linewidth, output power, and frequency stability, and choose a narrow-linewidth laser with a higher cost performance.
Summary
In conclusion, narrow-linewidth lasers and single-frequency lasers are both closely related and have clear differences. Narrow linewidth is a performance indicator for measuring the purity of a spectrum, while single frequency is a working state that describes the number of modes.
When choosing a product, consider not just the name but also the physics behind its technical specifications and consider the specific needs of your application. The SMART SCI&TECH Ultra-narrow Linewidth Single-Frequency Laser module is a semiconductor laser module that features ultra-narrow linewidth, high optical power, wavelength stability, low noise, and ease of use. If you are interested, please contact us for more information.
