## 1 Introduction  
A stroke occurs when partial blood supply to brain suddenly stops or when the blood vessels of the brain burst, consequently spilling blood into a space surrounding the brain cells which die when they no longer receive oxygen and nutrients from the blood or when there is sudden bleeding in or around the brain. Stroke has two forms: (1) ischemic blockage in blood vessels of the brain, and (2) hemorrhagic stroke that causes bleeding in the brain. Globally, stroke is the second-largest cause of death and third-largest cause of disability [1]. In 2019, a number of global stroke patients reached 115 million. A number of ischemic stroke patients was 77.2 million, a number of hemorrhagic stroke patients was 20.7 million, and a number of stroke patients due to subarachnoid hemorrhage was 8.4 million [2]. Also, 3.3 million people worldwide died from stroke in 2019.  
To look a little further at the progression of stroke in the cell, tissue, and brain function, first, clogging or bursting blood vessels results in irreversible damage to the center of the brain tissue directly associated with it. After that, brain damage and penumbra occur in the surrounding brain tissues. In neuronal cells of tissues damaged by stroke, glutamate is released from ion channel collapse and excessive inflow of calcium occurs [3, 4]. In addition, energy production decreases due to a damage of DNA (deoxyrjbonucleic acid) [5, 6]. As a result, both necrosis and apoptosis of neuronal cells occur in brain tissues damaged by stroke. Also, blood corpuscles are accumulated by factors such as interleukin 8 (IL-8) expressed in damaged brain tissue by stroke [7]. It enhances an inflammatory response of the brain tissue. In such damaged brain tissues by stroke, neurons and blood vessels are regenerated and recovery proceeds for weeks to months [8]. Studies to understand anatomical and functional brain damages related to complex connectivity and study functional recovery mechanisms due to neuroplasticity after stroke have been actively conducted [9; 10; 11]. Research, development, and medical applications in the diagnosis, prevention, treatment immediately after stroke, and rehabilitation/therapeutic techniques for post-stroke patients have extensively progressed for high incidence and mortality of stroke and studies on mechanism of stroke, recovery processes, and therapeutic techniques developments.  
Biomedical optics is a research field that studies both the principle of light-biological specimen interactions and development of medical applications [12; 13]. Research and diagnosis techniques using light have advantages such as lower invasiveness and the ability to obtain varied biological and medical information compared to diagnostic modalities using radiation and other energies. Additionally, benefits of light-based therapeutic methods have fewer side effects and can be combined with conventional treatments such as chemical drugs. In particular, neurophotonics has been extensively used in studies using light for research, diagnostic, and therapeutic modalities in brain and nervous system diseases [14; 15].  
In this article, we explored optical techniques for research, diagnostics, and therapeutic modalities for stroke and related brain injuries. Specifically, three detailed techniques are addressed: (1) photochemical thrombosis to establish stroke animal models with localized brain lesions; (2) optical imaging techniques for in vivo preclinical studies for understanding stroke mechanism and developing diagnostic or treatment modalities; and (3) optical neurostimulation based therapy of stroke and preclinical and clinical assays related to it; and. An exploration, an identification, and an analysis of previous studies of optical modalities for stroke are confident that they will help with the processes from studies to clinical applications of optical methods for research, diagnosis, and treatment of stroke.